What exactly is slow when you use GLUT?
I'm just curious because all GLUT does (or should do) is handling the window system stuff in a platform-independent manner.
As long as you stay clear of the menu/widget/button "GUI" part and the font rendering, your games should run fine (and still be portable).
If that part was what you where looking for, however, you're now screwed, of course. But IMHO, you should definitely try to create your own GUI toolkit (which can be tightly coupled to your render code, and it's fun!), or try to find some lightweight OpenGL GUI library on the net.
As for Qt and GTK: Both are very powerfull and versatile, and Qt would certainly be my first choice for a tool or an application. But I think these libraries are more than a little bit over the top for a game...