[popcornmix]

Overheat after startup kodi checks my Nas for new files and the heat symbol appears.

I assume this is a Pi3? What is ambient temperature like, and is the Pi cased?
Lack of ventilation can increase chance of high temperature. A Flirc case (including Kodi's branded version) is very good at reducing temperature.

The Pi3+ also runs much cooler. The warning is not a real issue, it just indicates that turbo mode is being limited due to temperature.
Apart from not running as fast as possible it is harmless. You can disable the warning with "avoid_warnings=1" in config.txt