I can confirm this strange result when using the US International PC keyboard layout, but I can also confirm that using the standard US one works fine. If there's no pressing reason you need to use US International PC, I'd suggest simply using the standard US one instead, but if you do need to use US International for some reason, the Set Action Delay action set up like this before the clipboard insertion action appears to resolve the issue on my system with minimal (though still noticeable) slowdown (in other words, it's most likely a timing issue as you suspected, though why it only presents itself with this keyboard layout and not the standard US one is beyond me):