Weird way to improve performance

So everything in LSPDFR works perfectly fine, except for one thing: when I first start the game it runs smoothly about 60-65 fps which is perfect. But sometimes drops suddenly at 40-45 fps, and there's a slight chance to revert back to 60. But I figured out a way to fix this thing more quickly instead of waiting. I press the windows tab for the desktop to show up, open task manager and close it, and to a desktop refresh a few times. This for some reason will boost my game performance, but unfortunately only for  2 or 3 min or less. I have tested if driving fast with a lot of population and trees will affect it, it won't. Any ideas how to fix that for good?