I launched PL SQL Developer version 15 release about 1 hour ago and it hangs for several seconds to minutes up even when I mouse over a scroll bar. Trying to scroll is impossible. In the Task Manager, power consumption for the PL SQL Dev process goes to "Very High" during the hangs.
I have clipboard history turned off and it still hangs up.

I like the dark mode though! Thanks for that! Please fix performance problems before I have to go back to v14

To obtain some more diagnostic information, can you modify the PL/SQL Developer shortcut and add the DebugSQL parameter? For example:

"C:\Program Files\PLSQL Developer 15\plsqldev.exe" DebugSQL

Reproduce the problem and send me the debug.txt file that is generated in the %APPDATA%\PLSQL Developer 15 directory (e.g. C:\Users\<User>\AppData\Roaming\PLSQL Developer 15).

I also am noticing sluggish performance in v15. It takes about 6 seconds on startup before the login dialog appears. It takes 4 seconds to bring up the Preferences dialog. And so on. My PC is older(ish) but still not a weakling (HP ZBook Studio G4, 64-bit, 4 cores, 32 GB RAM, solid-state drive).