Im old. It used to be derogatory against imported cars to North America. Rice Racers meant Japanese imports that were modified.

But the meaning or rather the connotation has changed. It now is more related to the cooking term of ricing, where you pass a vegetable through a ricer to break it into rice sized pieces. You rice your PC by tuning all the pieces and making minute tweaks.

As another commentor added the RICE term for cars is now a backcronym of Race Inspired Cosmetic Enhancements