Rewriting VFP (or any application for that matter) would not by default speed up VFP. If you develop a bad alogirthm then irrespective of the development language your code will be slow. It doesn't hurt to use assembly language for critical sections of code (example would be memory copies) but modern compilers do a damn good job (some better than others) of generating code.

A good example of this is the games development world, most games and their support tools are written in C/C++ with critical sections being optimised assembly language.

Good assembly language resources can be found at http://webster.cs.ucr.edu/ or try reading some of the graphics stuff by Micheal Abrash (Quake god!)