As always, depends on the implementation.
Hash maps should generally always have faster lookups than Btree based structures. However, they'll have slower writes especially when contested. A key issue hash tables have to deal with is what happens when a remapping needs to happen. For example, when 2 keys have the same hash. In that case, locking becomes a lot more messy.
For a btree this is simpler. It's built to be able to handle reshuffling and rebalancing in a way that's semi easy to have fine grained locks around.
Hm yeah, now I'm wondering how MySQL implements it. Consistent hashing is one way to limit the scope of a remap.
MySQL docs claim that a hash index is much faster if you only need kv lookups, so it seems like uuid4 with hash index would be suitable. Never tried it though, and can't say whether it's faster than using a btree with uuid7. Seems like in theory it would be.