It has been one month now since my game has been running slow ; I originally thought the issue was my computer that was aging. But I recently bought a new one (not because of KSP of course but because it was quite old) and first thought everything would be resolved : in fact it remained quite the same ; at least my computer doesn't tend to overheat like my previous one.
Basically, when I say KSP is running slow, I mean the mission timer is ALWAYS yellow, at least until I get into space, and even up there, the timer is often yellow. The time passes by 2 to 4X less than its normal rate, making it somewhat challenging and upsetting to fly my ships. I apologize in advance for the possible mistakes as I'm French. Here's my KSP folder :