ViaBTC | Zcash NU7: What Will 25-Second Block Times Mean for Miners?

Zcash NU7: What Will 25-Second Block Times Mean for Miners?

2026-10-13 18:01

Zcash's next network upgrade, NU7, proposes reducing the block target spacing from 75 seconds to 25 seconds. As of October 12, 2026, NU7 is active on Zcash's public Testnet, but its deployment specification remains a draft and its Mainnet activation height has not been assigned (ZIP 259). The headline figure — three times as many blocks per day — invites an intuitive but incorrect conclusion: that miners will earn three times as much scheduled subsidy revenue. The protocol proposal does not work that way. This article explains what actually changes for miners under 25-second blocks, and what does not.

The Arithmetic: Why 3× More Blocks Does Not Mean 3× More Revenue

At a 75-second target, the network produces a nominal 1,152 blocks per day (86,400 ÷ 75). At a 25-second target, that figure rises to 3,456 blocks per day (86,400 ÷ 25) — exactly three times as many. However, ZIP 218 also reduces the scheduled subsidy paid per block by a factor of three and increases the full halving-interval parameter from 1,680,000 blocks to 5,040,000 blocks. If activation occurs partway through a halving period, the remaining portion is rescaled according to the activation height; it does not restart a full interval. These adjustments are designed to preserve the approximate time between halvings.

The result, subject to zatoshi rounding, is that scheduled issuance per unit of time is intended to stay broadly unchanged: 1,152 blocks at the pre-NU7 subsidy produce approximately the same total ZEC as 3,456 blocks at one-third the subsidy. At the network level, this means more frequent blocks with smaller scheduled subsidies, while the aggregate scheduled issuance rate remains broadly unchanged. It does not mean a pool miner will receive account credits or withdrawals more frequently: those follow the pool's settlement and withdrawal rules, rather than the block target alone.

This point applies to scheduled subsidy only. A miner's realized revenue also depends on network-wide hashrate, pool fees, pool luck, transaction fees, rejected shares, uptime, and the pool's payout method, none of which are fixed by the block-time change itself.

What Actually Changes for Miners

Block Propagation and Template Turnover

A shorter target interval compresses the time available for block-template distribution and propagation. Under a 25-second target, a pool or miner has roughly one-third as much time, on average, before the next block is expected, compared with 75 seconds today. This raises the practical importance of low-latency Stratum connectivity, prompt work updates from the pool, and well-connected pool nodes — not because the protocol mandates new miner behavior, but because the margin for delay narrows.

Research on Zcash block verification has found that verification time depends on transaction composition rather than block size alone, which is one reason propagation and validation speed matter independently of how large blocks become (Stiehle et al., 2021). NU7 addresses part of this concern with a global shielded-action budget of 330 actions per block, intended to cap worst-case per-block validation cost even as blocks arrive more frequently.

Stale Blocks vs. Rejected Shares

It is important to keep two distinct concepts separate. A stale block (also called an orphaned block) is a valid block that is not retained on the winning chain, typically because another block at the same height propagated and was accepted first. A rejected share, by contrast, is a submitted unit of work that a pool does not credit, which can result from communication delay between miner and pool, stale work, or hardware issues such as overheating or outdated firmware (ViaBTC support documentation).

ZIP 218 estimates a theoretical stale-block rate of approximately 3.26% at a 25-second target, derived from measured Zcash propagation delays. A separate experiment on a 99-node, geographically distributed Zebra devnet producing 2 MB blocks at a 25-second target observed a 4.86% stale-block rate and a 0.37% fork rate. None is a post-NU7 Mainnet measurement or a forecast of any pool's rejected-share rate. A pool's actual stale-block rate depends in part on its node topology, peer connectivity, and block-validation and propagation performance, which differ from the devnet's conditions. Rejected shares instead need to be assessed through miner-to-pool connectivity, work-update delays, and invalid work. These network-level estimates and observations are reported in ZIP 218.

Difficulty Adjustment After Activation

NU7 proposes lengthening the proof-of-work averaging window from 17 blocks to 102 blocks. At a 25-second target, 102 blocks span 2,550 seconds (about 42.5 minutes), compared with roughly 1,275 seconds (about 21 minutes) under the current 17-block window at 75 seconds. The window is lengthened in block count more than proportionally to the change in block time, which is intended to avoid making difficulty respond more erratically simply because more block heights pass each hour. Miners should expect observed block intervals and difficulty to take some time to settle immediately after any Mainnet activation, particularly if network hashrate shifts around the same period — a pattern with some precedent from Zcash's 2019 Blossom upgrade, which reduced target spacing from 150 to 75 seconds and saw its 20-block rolling average reach approximately 74 seconds within 64 blocks of activation.

Separating Block-Time Effects from NU7's Other Reward Changes

NU7 bundles several consensus changes beyond block timing, and conflating them with the 25-second target risks misstating miner economics. ZIP 235 proposes removing 60% of transaction fees from circulation, leaving 40% directed to miners — a change to fee treatment that is independent of block frequency. A related proposal, the Network Sustainability Mechanism (ZIP 237), would reissue previously accumulated value through future block subsidies while preserving the overall supply cap; this is described as an addition to scheduled subsidy rather than a consequence of faster blocks. Under ZIP 259, reissuance begins at a separate, later height, not at NU7 activation. Miners should therefore not treat it as an immediate offset to the change in transaction-fee allocation.

The accurate framing is therefore two-part: the 25-second block-time change (ZIP 218) is designed to keep scheduled issuance broadly unchanged per unit of time, while separate NU7 proposals (ZIP 235, ZIP 237) independently alter how transaction fees and reissued value reach miners. Evaluating post-activation miner revenue requires accounting for all of these rules together, not the block-time change alone.

Evidence From Testnet and Devnet

The Zcash Foundation's October 6 engineering update confirms that NU7 activated on public Testnet at block 4,465,026 on October 4, 2026. This allows the 25-second block rules, the revised averaging window, and the shielded-action budget to be exercised under real network conditions rather than only in specification form. Activation establishes that the upgrade has entered public Testnet operation; it does not by itself demonstrate that every mechanism has been validated as operating as designed. It also does not establish Mainnet propagation behavior, Mainnet hashrate distribution, or Mainnet stale-block rates, which depend on factors — node geography, peer count, miner concentration — that differ materially between Testnet and Mainnet. Readers should treat Testnet activation and devnet results as evidence of testing progress, not as a profitability forecast.

A Practical Monitoring Checklist

Miners and pool operators preparing for a 25-second block target may find it useful to track the following around and after any Mainnet activation:

Rejected-share rate: Compare the pool-reported share rejection rate over a consistent time window, keeping it separate from stale-block statistics.
Uptime and connectivity: Monitor worker uptime and connection stability.
Hashrate: Compare ASIC local hashrate with the pool's estimated hashrate over matched periods, recognizing that they are related but distinct measurements.
Payment-method rules: Confirm the ZEC payment method available on the chosen pool and how its rules affect exposure to pool luck, transaction-fee income, and settlement timing.

Because payment-method availability and terms can change, miners should confirm current ZEC rules in their pool account settings or with pool support rather than relying solely on earlier documentation.

FAQ

Will miners earn three times as much because NU7 triples the number of blocks per day?

No. ZIP 218 reduces the scheduled subsidy per block to roughly one-third of its previous level at the same time it triples block frequency, so scheduled issuance per unit of time is designed to remain broadly unchanged, subject to rounding. Realized revenue also depends on hashrate, fees, pool terms, and other factors not fixed by block timing alone.

Does NU7 change how ZEC issuance is scheduled over the long term?

It changes the block-count parameters while preserving the approximate time-based halving schedule. The full halving interval increases from 1,680,000 to 5,040,000 blocks. The remaining portion of the current period is rescaled according to the activation height, so activation does not start a new full halving period.

Is a stale block the same thing as a rejected share?

No. A stale block is a valid block that fails to remain on the winning chain, measured at the network level. A rejected share is a pool-level outcome affecting a specific submitted unit of work, and it can have different causes, including communication delay or hardware issues.

When will NU7 activate on Zcash Mainnet?

As of October 12, 2026, NU7 is active on public Testnet, but its Mainnet activation height is listed as unassigned ("TBD") in the deployment specification. The specification says the Mainnet height is to be set on October 20; that is not a Mainnet activation date. Miners should not treat Testnet timing as a Mainnet activation date.

Does ViaBTC support ZEC mining, and will NU7 change the payout method?

ViaBTC supports ZEC mining. NU7 changes network consensus rules; it does not itself determine which payment methods ViaBTC offers. ViaBTC's January 9, 2026 announcement confirms the introduction of PPS+ for ZEC. For currently available options and fee terms, check Pool → Mining Settings → Payment Method or contact ViaBTC support, and consult any upgrade-specific announcements before assuming a change.

References

Binance Pool Alternatives: How to Choose Your Next Bitcoin Mining Pool

How Will Zcash NU7 Affect Mining Rewards and Transaction Fees?