I'm actually sort of impressed how well JEdit performed. Other than micro its one of the only not written in a C/C++ language (the critical components... I realize emacs, code, atom, sublime all have their extension languages).
Java virtual machines generate very efficient machine code nowadays, especially for an older codebase like jEdit's that is not particularly "object oriented".
I think it would be more fair to call it object oriented. It's just not framework oriented, and that's a good thing.