I have a relaively large Qt/C++ project containing many files. When I open the project .pro file for the first time, it takes a long time before I am able to select my appropriate kit. The progress bar at the bottom right shows that it's reading my project. After a minute passes I can uncheck the selected kit and select another one.

It was not like it in Qt Creator 3.3.0. It's so annoying because if you have many large projects, whenever you want to open and configure one, you should wait for atleast 30 seconds before the Qt Creator GUI is responsive.