As always, depends on the implementation. Hash maps should generally always have faster lookups than Btree based structures. However, they'll have slower writ

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.