>>716257450These things are old enough that it could've developed a cooler mounting issue, there could be compacted dust blocking heat transfer in the heatsink (unlikely if the issues arose suddenly), one of the cooler fans could be stuck/slow, the thermal paste could've failed etc.
Technically 90c is safe for an RX580, but the lower FPS sounds like the card is throttling to avoid overheating. If it ran at the same clocks as it used to in its current state, it'd scorch itself.
It could be worth taking it apart, inspecting the insides (heatsink, pipes, transfer surfaces) and reapplying the paste. If you do so, keep in mind that pasting a GPU isn't quite like pasting CPU. GPU dies are bare without heat spreaders, so the paste has to cover the entire die (the pea method could lead to a burnt GPU here). The cooler has to apply even pressure so the die doesn't crack, so take care screwing the cooler back on. Also make sure none of the VRM and RAM thermal pads go missing.