Hey!
I’m having the same FPS drop issue. In my case I get less FPS when stretching the game, for instance when I like to play on full screen. For the most FPS you shouldn’t stretch your client.
Secondly, If this doesn’t work, you could always play through a Steam client or even some kind of 3rd party client, for example hacked clients. I don’t recommend playing with hacks, but it could be a solution. You could always disable all hacks to play fairly (as you should).
I hope this helped!
With kind regards,
IIjellekeu