I've been playing online FPSs for years now, but I've always been absolutely terrible at them. I can maybe get a 1:1 K:D ratio on a good round, but my average is probably closer to 1:3 or 1:4. This has been pretty consistent in every FPS I've played over the years. I'm not sure what I'm really doing wrong. I use cover. I try to flank with my teammates. I throw grenades to flush enemies out of cover. But somehow it always seems like they're faster, more accurate, better able to get the drop on me.
Is it just because I only play them for a few weeks or months before moving on? Is this something that gets better with practice, or do I have some kind of undiagnosed motor deficiency?
Mouse sensitivity! Turn it down as low as you can go. Buy a large mousepad and use your arm. If your mouse has a sniper button, make it actually SPEED UP your sensitivity instead of slowing it down. You can use that for close quarters.
Framerate! Make sure your settings are low enough to give you at least 60fps, preferably 120
Monitor lag! Make sure your shit isn't in like HDR movie mode.
NEVER use wifi
What games do you play? There are some broad types of FPS games that have different playstyles.