weird low fps issue

[gearsman]

I have old pc but doom bfg shouldnt have low fps. I have amd phenom 9600 2.31 ghz x4, 3 gb of ram, hd radeon 6670 1 gb of ddr3 vram directx 11. I get 120 fps with hl2 maxed out, decent fps with bdl2 maxed out. skyrim maxed out to me still has better fps then doom 3 bfg. Is it just another game with fps issuse like csgo? out of all the games I play doom bfg and csgo have the most poor fps issues and they are the most low graphed games I play on this pc.