When players commenting about desync in games (opponents see you and shoot at you before you can see them) usually tips are to use TCP optimizer. Most games use UDP protocol so improving TCP can only make things worse and if there is improvement its short term placebo. Quality of internet connection is most important because if you have poor ADSL/VDSL connection with a lot of buffering on DSLAM side there is nothing you can do about it, you are doomed. If your internet connection is good fiber/cable fast speed non buffered and you still have desync problems here are few things you can try to do:
1. Make sure you play on low ping servers all the time (on high ping servers you will experience desync no matter how good is your internet connection)
2. Change Receive and Transmit buffers in your network adapter settings until you really feel that you have less desync. Intel NIC still are best for gaming so avoid motherboards with Realtek NIC
3. Use XMP profile to boost your memory speed also 1:1:1:2 memory ratio work best for gaming and help to minimalize desync so when you are buying new memory look for kits with such ratio.
4. If you have open WRT router make sure you prioritize UDP traffic
5. Many fast paced FPS games use prediction for your interactions/position if server not receiving your packets in real time so make sure there is no buffer bloat on your side caused by torrents/ high bandwidth traffic in the background while you are playing. When game predict your position it can render your player model to be visible to your enemy even you on your screen are in the cover at that moment.
6. Some players claim that poor grounding of PC can be also cause of desync in games, my powerline is well grounded so i would not know but its worth to check though
happy gaming and see you on the battlefield soldiers :)