A few years ago I wrote about my Problems with CDB Debugger in QtCreator. And a few months ago debugging started getting slower and slower and it finally reached a point where the program would need a minute or more just to start and stopping for a breakpoint was almost a coffee break situation. The old remedy wasn't helping anymore — this was something new.
While searching for a cure I stumbled upon Qt mailing list thread where the culprit was identified. It's Microsoft's symbol server!
Just go to Options -> Debugger -> CDB Paths tab and make sure srv* path is below cache* (or just remove it altogether). Looks like server symbols are preferred over local symbol cache if on top and that server is now terribly slow.