Hi there!
PC specs:
I5 4570, GTX 660, 16 GB RAM 1600 MHz, SSD Crucial 480 GB, AeroCool 650W
Last version of Nvidia drivers, have nothing in Auto download
I started to play Forza when I had 8 GB RAM and slow HDD. I played with low-med settings, but high shadows and I had about 45-60 FPS with constant microfreezes. I decided to buy solid state drive and expand memory storage. At first I moved my old system to SSD without installing new one. PC started slowly but all the games ran fast ( besides Forza). I deleted my old system and installed new one. On totally new system game runs awfully, but OS starts normal. I set settings to the lowest and I have at start 40 fps and then it drops to 13-14. When I put med or high it still 14 fps. Running the game I noticed that it starts to downloading something in the main menu for a minute and only after this it writes my nick and I can play. GPU works on 100%, CPU 60-80%. Also I have some problems with The Witcher 3, FPS was 40-45 and now 25-30. I tried all the methods which were the solution for another gamers.
In a month I’ll buy RX 590 and I hope that it will solve my problem but still I want to play the game which I payed for right now…
Help plz 
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Forgot to say that I did BIOS reset