for anyone looking for details on the "slow to load code in user.clj" problem with latest jdks - see https://dev.clojure.org/jira/browse/CLJ-2484 and https://github.com/puredanger/slow-user-load. Probably best to take any serious discussion to the jira comments.

I guess map merge could be made faster by using transients here? https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/APersistentMap.java#L40

APersistentMap is an abstract class extended by many map implementations, and generically they can't all be made into transients, only those that are ITransientAssociative. If you are using a transient map already, then you should get that effect (via the impl in ATransientMap). The concrete collections (which know whether they are transient-able), like PersistentHashMap / PersistentArrayMap, could probably override that function and do that though.