X under heavy load

[Marc Brünink <mbruen@smartsoft.de>]

Hi,

if I put my 500 Mhz machine under heavy load, X doesn't respond 
anymore. Actually it works, but it's much to slow. I want to do some 
hard calculations in background AND use X. I guess this should be 
possible, shouldn't it?
I tryed to use nice 20 heavy-load-process but this didn't help a bit. 
Is process scheduler of Linux just crap? Is this a problem of X? What 
should I try next?

Thanks
Marc