Ever wondered why it takes years (exaggeratingly) to input a single character in chat or even scroll down/up just by a bit? And also, why do notifications take so long to pop up yet so long to disappear?

Well, here is a "fix":
On your FiveM main menu, click "Settings", otherwise known as the "FiveM cogwheel" shown below:


Then, try to find the setting "NUI in-process GPU" and enable it. Also shown below:
 


Here is the shortest version of why: Things start to get shakey when your GPU is used to 100%.

To explain more on a shorter version of why: FiveM has multiple processes under it, each with a lower priority than the other. Chromium, one of FiveM's sub-processes that usually handles UI, needs to use as much of your GPU as necessary to function well enough. But when it can't use as much as it needs (since the main process has priority), it starts to compensate for the lack of GPU time (under the form of "lagging").