Performance measurement: cost of the mempool txid check

[tvpeter]

Description

This PR seeks to provide the performance metrics for adding compute_txid() verification check for every
fetched mempool transaction to Emitter::mempool_at
in #2283.

Tests

• crates/bitcoind_rpc/tests/perf_check.rs — an end-to-end test using bdk_testenv (spins up a real regtest node), which broadcasts ~500 mempool txs and times get_raw_transaction (the fetch already done per tx) against the
  compute_txid() check.
• crates/bitcoind_rpc/benches/txid_check.rs — a criterion microbench isolating the CPU cost of compute_txid() vs a bare consensus-deserialize, across three transaction sizes. Provides the absolute cost.

How to reproduce

• Run cargo test --release -p bdk_bitcoind_rpc --test perf_check -- --nocapture for end-to-end test module
• Run cargo bench -p bdk_bitcoind_rpc --bench txid_check for criterion benches

Results

Machine: Macbook pro M1 (16GB RAM, macOS Tahoe v26.6.2, 10 Cores CPU).

End-to-end (500 mempool txs, localhost regtest):

	per tx	share of fetch
get_raw_transaction (RPC round-trip + hex + deserialize — already paid)	~117 µs	100%
compute_txid() check	~1 µs	~0.85%

Isolated CPU cost (criterion, time: estimates):

tx shape	compute_txid (the check)	deserialize
small 1-in/2-out (223 B)	~637 ns	~263 ns
typical 2-in/2-out (372 B)	~823 ns	~362 ns
large 10-in/10-out (1.8 KB)	~2.69 µs	~1.43 µs

Interpretation

• The check costs a stable ~0.6–2.7 µs/tx (size-dependent; ~0.8 µs typical), scaling linearly with tx size. Criterion reports "no change / within noise" across runs, so it's reproducible.
• It's ~2× a bare deserialize, but both are sub-microsecond — and it's under 1% of the full get_raw_transaction fetch, which is dominated by the RPC round-trip.
• Extrapolated to "thousands of txs": at ~0.8 µs/tx, a full 100k-tx mempool is ~80 ms of hashing on top of a fetch phase measured in tens of seconds (localhost) to minutes (remote). Sync is RPC/IO-bound, not hash-bound.

Conclusion: the txid check has no meaningful impact on sync performance.

Checklists

All Submissions:

• I followed the contribution guidelines