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.
Ive cleared cache -> nothing improved;
Ive been looking throughout the internet and it seems that many people have similar problems, there are many solutions to this given but none works 100%......
I think that Apple messed sth with performance but they just worsened it instead of improving it.
I even installed Chrome:: works well.
Will wait till Apple releases update/patch to Safari.