I bought XFX RX 570 8GB little over 3 months ago, it worked great but recently I noticed that games lagged a lot. Same settings on same games as when I first bought it. I'm running the latest AMD Radeon Software driver.
is this tjunction temp ? if it is then no problem , but if it's the "basic" temp , yeah maybe a little high ...
you can try to manually change the fan curve for example try to set it at 100% after 75-80° to check, but if this doesn't help maybe you have to change thermal paste
if card is too hot it can auto throttle to try to cool by autolowering it's perfs until temps are cooler
i just checked on my other computer with rx 570 on it ... yeah in fact there is no tjunction temp with 570 apparently ... so yeah i think you problem can probably be a thermal throttling problem .. increasing fan curve (if not already done) could help
then if this solves your problem you can try to find a way to increase air flow in you pc case to be able to lower the gpu fan curve back if you need