Issue appears to have fixed itself however as I mentioned in my last comment I believe it may have to do with hud elements rendering in the background.
You can see in this video a steady decline in performance if I hold my mouse inbetween classes (for god knows what reason). The frames steadily decline and when the game goes to render that hud element again it continues dropping from where it was previously. Hud_reloadscheme doesn't fix the drops. Current theory is that when MGE forces you to switch to a certain class it may somehow trigger the class menu hud element but honestly I have no clue.