Is you Kodi build big and do you use a skin with widgets?
Also have you tried force closing running apps in the background?
I have seen users with a similar complaint on Reddit and Kodi forums.
Some mentioned factory reset, rolling back the firmware, disabling certain features and just using a basic setup on Kodi with standard skin and a couple of addons.
Obviously the more you add to a device the more resources it needs.
Apps custom launcher.
Another user asked the same question but didn’t seem to get a definitive answer. Nor did they say if how they fixed the issue.