I like to play Team Fortress 2.
My computer can run it fine on the recommended settings, but I like the best performance possible. I use a custom configuration that makes the textures look like they are from late 90s, but my game performs very well, which is what I am after.
One thing that I noticed though, is that couple hours into long sessions, the performance starts to deteriorate. I have my framerate capped out at 60 (good number; it's capped because I like consistency), and for the first couple hours it stays at 60 even during intense fights. After the first couple hours though, it starts to decrease. It would dip into the high 30s - mid 40s when I get into fights.
If I were to restart the computer, I would get another couple hours of steady 60 FPS. What I don't understand is: why? How come a restart makes my game perform better, even though my system is not running at full capacity when playing it? What is the magical thing that happens during the restart that makes the computer pick up the pace and give better performance? Is there any way to achieve those results without a restart?