8 GB of RAM but the lag (Grand Theft Auto 5)

I have a laptop with integrated graphics 256 MB.
But the game says 3 GB (I know that onboard video card takes RAM) and poluchaetsya that I have 5 GB of RAM, which is not enough for the game and after 15 minutes lag
The QUESTION how to reduce video memory?
P. s In the BIOS no just do nothing
For even earlier thanks
Processor:Ryzen 3 2200u
Video:Vega 3