I noticed the same thing. I was just searching the forum for a solution and came across this thread.
I have been using PL/SQL for over two decades.
We recently renewed our license with a new version, but after a few hours of use I was quite disappointed with the new version.
Freezes are very common in the PL/SQL editor. Sometimes it's about delays that are measured in milliseconds, and sometimes it's about few seconds. When using Copy/Paste, it is more than obvious.
I will use older version and wait to see if there will be some fix in new releases.


Version:
15.0.3.2059 (64 bit) on Windows 10