Left 4 Dead 2 Lag

Mar 17, 2018
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My Specs:
i5 3470 3.2 ghz
Gigabyte geforce GT 730 2 gb, GDDR5 OC
8 gb ram
Why i play Left 4 Dead 2 in 40-60 fps in very low graphics and low resolution?
can anyone help me?
PS: I have vsync OFF
I play gta 5 on high 60 fps and i cant play l4d2 on more than 60 fps?
When horde is coming i have 20-40 fps, HELP :(((