Runs as fast as any 2D system can within Blitz3D.
Nope, it's not as fast as any 2D GUI can run in Blitz3D. It's slow.
The -fastest- would probably be one that uses a DLL to draw the graphics directly to the images and calculate all the neccessary information pertaining to events and such, and then probably poke events into a bank.
My 2D GUI draws neccessary bits into its own image (a buffer, so to speak) then draws visible portions to the screen. In doing this, you're not locking the back (or front) buffer as many times which Rect/Line/Text/etc. does in order to draw. Also, by doing this, I can have snazzy gradients and such- something that would be extremely costly for BlitzUI to use.
If you have the cash, buy F-UI 3D. It's 3D, really fast, and you can set the alpha of windows. So you could just create a window that's the size of your screen, and then set its alpha to 0, where only its gadgets would be visible. It's cheap, so getting the money for it shouldn't be difficult.