when u play NFS rivals minimize it & see open task manager & check the CPU usage...if its like 98% or 99% then this is the reason ur game is lagging...
also check the disk usage... sometimes games lag if ur disk usage is also more..
close all other application and also change ur windows theme to classic if u r using win 7 or 8 or 10.....then run ur game.. if still its lagging then definitely its because of ur processor...