New carset, new problems

Hi all:

I just downloaded a carset for a well known league, my problem is with that carset the game has a very high PO. Maybe could be this for the hi-res textures? I haven´t had any problem with the original wads, I have the settings at max and the game runs smooth, but when I install the new carset and the original wads are overwritten then I have problems even starting a race. How could I resolve this?
I used search with no results.
Thanks

I don't have a problem with hi-res textures, they only give me a 1% PO increase. Have a play around with the sound, as this adds hugely to the PO in my experience.