pkg/bench/hashbench: potential performance regression

pkg/bench/hashbench [performance regression](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Nightlies_RoachprodMicrobenchmarkWeeklyWip/21080195?buildTab=log) on master @ [fdb92776bd279bee489ced860ce37f6c003827aa](https://github.com/cockroachdb/cockroach/commits/fdb92776bd279bee489ced860ce37f6c003827aa):

Performance regressions detected in package pkg/bench

Comparison: v25.4.3 (71d85362) -> refs/heads/master (fdb92776)

Found 3 benchmark(s) with regressions ≥20%:

- [ ] [pkg/bench/hashbench→LatencyWhileHashing/hash=MD5/block=1024KB/file=16MB-32 (max-latency): +51.45% (51.4%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=LatencyWhileHashing/hash=MD5/block=1024KB/file=16MB-32&package=pkg/bench/hashbench)
- [ ] [pkg/bench/hashbench→LatencyWhileHashing/hash=MD5/block=1024KB/file=8MB-32 (max-latency): +49.94% (49.9%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=LatencyWhileHashing/hash=MD5/block=1024KB/file=8MB-32&package=pkg/bench/hashbench)
- [ ] [pkg/bench/hashbench→LatencyWhileHashing/hash=SHA256/block=512KB/file=16MB-32 (max-latency): +32.44% (32.4%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=LatencyWhileHashing/hash=SHA256/block=512KB/file=16MB-32&package=pkg/bench/hashbench)
Help

See also: [How To Investigate a Go Test Failure \(internal\)](https://cockroachlabs.atlassian.net/l/c/HgfXfJgM)

Jira issue: CRDB-60119