I compete in a lot of XC and Hunter trails in Yorkshire and I've never known a fastest time one.  It'd be too dangerous to encourage horses/ponies to go as fast as possible over the XC fences because they're solid.  They sometimes do arena XC which could be fastest time.  If you want to go quicker then the higher the jumps the faster the pace needed for optimum time.  When you compete in the opens its usually 450 or 475mpm which is a pretty quick pace anyway, and you would need to be moving on for the whole course.  What height are you jumping at the moment?