Thermals displays your device’s thermal state, as well as how many processors are active on your device and its uptime, as reported by iOS.
• Slow charging — battery charging causes a lot of heat and iOS can limit charging speed to reduce the additional heat that might make a device too hot.
• Low maximum brightness — if your device screen stays dark despite being on full brightness, it could be limited by iOS due to heat.
Phone running slowly, charging slowly or low brightness? See if your device is overheating using Thermals.
Also an independent Apple Watch app shows the thermal status, active processors and uptime, and thermal history of your watch.