system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can online games like first person shooters be so in sync across all users without hardly any delay?
gpt
Fast internet and faster servers.

Human reaction times are on the order of 100ms or so. Ping time, which is round-trip packet flight time is often on the order of half that or so.  Round trip is therefore on the order of human reaction if you give the server a little time to do stuff. 

Also, the server only sends back just enough to run the game.  For example it may just send back Headshot and some points, and your client has to take care of the headshot animation, credit, sound and then tell the server when it's done because it's not really sync-critical.