This bounty requires resurrecting an existing PR that has fallen out of date. The PR optimizes use of the `ParseHex()` utility, which converts a string to a vector of bytes.
Hex string decoding is used a number of places in Bitcoin Core, for example in transaction deserialization.
The changes as-written improve the performance of this decoding by a factor of two, and adds benchmarks demonstrating the performance improvement using base64 decoding.
In order to claim this bounty, you should open a replacement PR with the same or similar code changes after rebasing. You should mention the original PR in the description of the replacement PR.