The speed of live view simply comes down to CMOS vs. CCD. Right now CCD sensors still produce better final net image quality, but they suck at doing live view. CMOS are great at live-view and capturing video but are not quite a match for CCD in final image quality. In the future that may change and if a medium format CMOS sensor can be made that match CCD for final image quality then I'd put my money on Team Phase One (Phase/Leaf/Mamiya/Schneider) making use of it first and best.
It has little to do with CCD vs CMOS. You get faster refresh rates with 35mm
DSLR cameras because they use line skipping techniques in order to get the data out and processed quickly. Also the processors are much faster.
Just look at how the D800 can process 1080 video. Scale it in real time for the on camera screen and out put uncompressed HDMI. That is massive data throughput . MF digital backs don't come close to these speeds.
Nikon D800 stills 36 MP at 4 frames per second .... no comparison.
Canon 1d X 18 MP at 10-12 frames per second.... no comparison.
As far as quality goes there have been significant improvements in CMOS, while CCD have not changed in this last generation, just a little bit more functionality. Fuji's new sensor in the X-pro 1 is simply amazing considering it's size. At it's resolution nothing out there beats it. If Fuji scales it up to larger formats it will be the standard to beat. The x-pro 1 at half the size already looks as good as or better than most full frame DSLR camera.
Fuji may well scale the sensor up to a FF 35mm size or even medium format.
After all Fuji already has the lenses and all the MF know how and above all in house sensor design and manufacturing, as well as a history of making CCD and CMOS sensors.