FPS drop problem after installing Proper Fixes

I started to install some mods in order to make SA better. After installing essentials pack, I install Proper Fixes with SkyGFX and Open Limit Adjuster. However, I encounter FPS drop problem in the game. For example:

https://preview.redd.it/5bx03hauc7og1.png?width=1920&format=png&auto=webp&s=8d3f0aef720bf4cf980a67af30468bfaa872797d

When I look at Downtown from Grove Street, my FPS drops from 60 to around 30

https://preview.redd.it/r59gtb8xc7og1.png?width=1920&format=png&auto=webp&s=f92ff386fb69c766a7c28232aa62dea44c1a52d5

It becomes more severe as I ride closer to Idlewood.

I just have four mods now except those in essentials pack: Proper Fixes, SkyGFX, Open Limit Adjuster. I wonder if I did not set the mods correctly. Please comment if you have any idea or advice. Thank you!