Unfortunately we kn ow nothing about your system to even hazard a guess as to why this might be.
e.g. OS, CPU, RAM, Graphics card/chip, etc. ?
Not to mention has this always been the case, and if not when did it start (and what changed around that time) ?
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?