FPS becomes lower and lower after each update

[aerosoul1337]

My rig:
Windows 7 64-bit SP1
Intel Core i7 4790K @ 4.00GHz
EVGA GeForce GTX 980 SC ACX 2.0 4GB DDR5 (GPU@1266MHz / Memory@1753MHz / Boost@1367MHz)
RAM 16.0GB Dual-Channel DDR3 @ 933MHz
Resolution: 1920x1080@60Hz

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My FPS becomes lower and lower after each update, specifically, started from "The specter of the rail".

Before that, my FPS in the liset could hit over 400 easily with max settings if I didn't cap it.
And in a full team LoR raid, I used to have 150~200 FPS with max settings.

After "The specter of the rail":
Normal mission (4 members): 120~160 FPS

After "The silver groves":
Normal mission (4 members): 70~120 FPS

After "The war within":
Normal mission (4 members): 40~70 FPS

Changing graphic settings doesn't help much.
Changing GPU driver doesn't help.