I was having the same problem and even reinstalled both the game and FiveM to now realize this is a problem with the graphics’ preference.
Solution:
Right click the FiveM launch file
Select the option Run with the graphics processor (at the very top of the list)
Select → High-performance NVIDIA processor (or whatever your graphic card is)
I realized this because of the notification that pops up on Windows saying that the resolution was changed to 800*600 by the igfxem module when i try to launch the game normally without FiveM.
There’s also an option to open FiveM with the Nvidia card by default on the same right click menu option. That way you won’t have to always right click to this preference.
Cheers!