Problem with my gtx 750 TI in GTA V

So here's my specs

Processor: 4 cores AMD A-8 7600 overclock 3,8 GHz

2x4 GB of ram

Graphic card gtx 750 TI


The thing is that even in the lowest setting 1080p the game drop under 20 fps in city.
However I can go play witcher 3 with high setting with smooth fps so I don't understand where the problem
 
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