Investigate places where we can use noncryptographic hash functions

We can now depend on System.IO.Hashing which provides noncryptographic hash functions
1. investigate where we're using cryptographic hashing inappropriately or custom hashing implementations
2. investigate performance benefit

Start by searching the repository for cryptographic hashing and custom hashing implementations, then review candidates that could use System.IO.Hashing. Measure the performance benefit for relevant candidates and document which changes are justified. Done means the candidate locations, measurements, and recommended replacements are clear.

Written by the indexing model from the issue text.