Have the same behavior on ipad pro 3rd gen. I see it mostly on eye dropper by double tap pencil 2nd gen. Generally cross show up like 0.2 sec (of course it’s not estimated but its quite fast), but once a 1-2 minutes there is significant longer time for eye dropper cross to show up like 1 sec or smth.
Also, if I work more then an hour or two, app become very unresponsive and laggy. So, I close it and start again. And it feels good again. So, kind of memory leaks maybe.