My PC’s performance dropped after cleaning it—what should I do?

I've been using an old computer that usually runs most games at 60fps without any lag. I cleaned it the other day, like I do every three months, because it generally boosts performance. But now, after the cleaning, I'm experiencing terrible lag, and my frames have dropped to around 45fps. I only used a blower and a quick-drying alcohol cloth. While cleaning, I checked the thermal paste on my CPU, and it was in bad shape. I'm considering replacing the thermal paste, but I'm confused because everything was fine before I checked it. How can I resolve this?