Question FPS problem in Runescape ?

[Soaray]

I installed a 2nd monitor today and am trying to play Palword on the first monitor whilst running Runescape on the 2nd one in the background, When I click into Palworld to play it, Runescape drops down to about 10-20 FPS, but as soon as I click back into Runescape it's FPS goes straight back up to 150.
Any idea on how to fix it please?

 

[rgd1101]

full system spec?

 

[Soaray]

AMD Ryzen 5 3600
NVIDIA Geforce RTX 2060
16gb Ram