Faster Frac() imlementation?

Hi all.. maybe you can help me. I'm working on a freeware realtime
audio application for the Asus EEE pc (a little notebook with a modest
630MHz Celeron M). I'm trying to optimize my time critical routines as
much as possible.. (why waste cpu cycles on things that don't improve
sound quality, right?)

For linear interpolation, which I frequently need to do, I typically
have to apply the Ceil and the Frac functions. The Fastcode Ceil
functions have helped me improve performance significantly..

Does something faster for Frac also exist? (My own silly basm
experiments all turned out slower than the RTL Frac :-)

Any help greatly appreciated. Thanks!
.