To be fair, “back in the day” (well before they reworked Forge Guard & got rid of it’s life on hit passive) I played a Shield Throw/Devouring Orb/Smite build that would easily get <1 fps on packs. It was glorious.
But that was a long time ago.
I’d imagine “20 fps at all times” is likely due to a pc on the lower end of the scale.