I think, you cannot do anything to improve to UI performance.
I use PL/SQL Developrt for a long time (since versio 6, or 7 maybe????) and the UI performance had a tiny drop in every version change. Not improvement, but a drop. The newer, more complex UI, menus, Ribbons, colored ribbons, themes , and so on.
And I think, it is because the software is written in Delphi, and the Delphi theme handling is not the best (maybe one of the worst )