I have a bit different problem.
1. When the PLSQL Developer Freezes It freezes also the whole desktop. I'm unable to click on any other program. When ver. 14 was freezing (due to recompiling many objects on base) then I could switch to another program. Now the whole Windows freeze. Looks like ver 15 is more connected with widows.
2. When I minimize and then maximize the PLSQL Dev it first shows black widows, then it begines to slowly refresh widows. It takes from 2 seconds to 10 seconds. Looks like that the longer it is PLSQL Dev out of focus the longer it takes to reassembe it. My computer is a few years old but quite strong, it should work better.