Saiten,
I have a C860 and pdaxrom ... I am no linux expert... I installed it, coupled with some applications ... and it RUNS TRULY SPEEDY ...
BUT THEN HERE IS THE EVENTUAL TRICK TO YOUR PROBLEM:
Are you installing into a SD card ? maybe your SD card SUCKS ! it takes LOADS of time to do the I/O Bussiness ... heh
I can tell that loading FIREFOX in my C860 takes some solid 20 seconds ... but if I look into the "CPU usage" bar, it is not at full (or event at 50%) most of the time: READING SPEED SUCKS in my Secure Digital Card .... I KNOW it I HATE IT, it slows down the whole thing.
On the other side, (not that it is such an overhead) I assume that applications fully grown, like the ones that pdaxrom can RUN once recompiled, with their tons of libraries, etc. are BIGGER, way BIGGER than the "Sharp" rom ones.
Other than that, C860 with pdaxrom feels quite decent on speed.... I think that pdaxrom is a fine tuned piece of code, crafted with care, which is getting better day after day ! So, saiten, please be more specific and technical about your rants and maybe we can find what is wrong in your case.