What problem does your feature solve?

LedgerSnapshot is used extensively in contract testing. Currently, entries are accessed and updated linearly. This is particularly expensive when a large set of entries is being updated in a large snapshot using update_entries.

• Slow CI — integration test suites that call env.ledger().snapshot() / update() across many contract invocations with large snapshots degrade non-linearly.
• Developer experience — no indication of why tests slow down as snapshot size grows.
• Fuzz / property testing — fuzzing harnesses that feed large generated snapshots can experience CPU saturation.

What would you like to see?

Accessing and updating ledger entries via a snapshot does not depend on the size of the snapshot, using something like HashMap or IndexMap.

What alternatives are there?

Leave it as-is. As long as snapshots remain reasonable in size, this is not really an issue.