I have been doing Codeforces (CF) for 6 months. Most of the time, I solve A and B problems of Div2 contests within 1 hour. I want to increase my speed and improve my ability to solve more problems during Div2 contests. For that, I want to practice only on CF. Actually, the problems on AtCoder and SPOJ seem harder and boring to me.

So, can practicing only on CF help me improve my problem-solving ability?