Step 1: Go the this file location C:\Users\"Name"\Documents\Assassin's Creed Syndicate

Step 2: Open the file named "AC"

Step 3: Change the following settings:

ReflectionQuality=2
Bloom=1
Fog=1
PostFX=1

Into:

ReflectionQuality=0
Bloom=0
Fog=0
PostFX=0

Now enjoy a smoother game!

I would recommend to first check out if you have enough FPS by only turning off bloom, if yes then keep it because the game will look pretty bad with all of those turned off. But of course if you really need the FPS then put all of them to 0 :)