It's possible that the update cleared some of the system caches, as it does some housekeeping during the install. Don't know if that would include the Safari caches, or not.
In any event, it would not hurt anything to clear Safari caches now, then restart Safari.
if the system rebuilds the cache files, you might experience some minor performance loss for a few hours, maybe a day or two at most. Give it a few hours.
I have experienced noticeably faster boots since the 10.14.3 update. It's very possible that Apple tweaked some of the system boot files. But, I don't know how that might relate to individual apps.