Please Backport r34202 and r34204 (rexml BaseParser uses instance_eval unnecessarily on listener add)
In add_listener in REXML::BaseParser, there's code to instance eval and replace the old "pull" method with a new one that calls all listeners. I assume this was done to avoid the cost of calling .each on the listeners when there's none registered, but when the list is empty this call is nearly a no-op anyway. The singletonizing effect of instance_eval and the redefinition of #pull, on the other hand, cause a cache flush throughout the system.
I have created a patch that removes the instance_eval, initializes the listeners array unconditionally, and does the event #each unconditionally. It passes all tests, and should perform better because it's not damaging the cache.
This can be safely backported to any Ruby version and has no visible behavior change.