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.