You may have had windows power settings set to conserve energy. In that case, windows would throttle back the cpu to reduce it's power consumption. Re-installing windows may have reset the power plan settings to basic. For rF2, I use the highest power plan settings: Ultimate.
And/or, you may have had a very cluttered windows with lots of old programs using memory and cpu cycles. Way back in the days of Win95, we wiped the HD and re-installed windows about every 6 months. That issue seemed to fade away. I wonder if it is something we should get in the habit of doing?