not sure what you mean by "speed up". More FPS? CTRL+drag one of the ends will speed up the clip. Preview playback? Either change the preview setting to something lower or use png's instead of tiff's. Tiff's are really slow to read because it needs quicktime (tga's are much much much much faster too).
Yep. That's what Tiff sequences do in Vegas. Word is that vegas relies on quicktime to decode Tiffs and the roundtrip is slow. The solution is to batch convert them all to PNG.