Okay, Felix told me to make a thread, and I will try to make a simple knowledge base in this thread by adding confirmed solutions.
Problem: FPS goes down after a while and makes the game unplayable.
Temporary Solution: Rename your FX folder to something else. Usually effects causes this, and this solution will remove any effects off the game. (e.g The lightning of paramun, green lines in arena)
1. Right click Ko4Life shortcut in your desktop, and click on Properties.
2. Click on "Find Path".
3. Click on FX folder 1 time and click on F2 and rename the folder to something else.
If you want effects to be enabled in the future, just repeat the same thing and rename folder back to FX.
In addition, you can do this on your old/slow computers to maximize your FPS.