From out of the blue I would think of thermal problems.
If you run a cpu stress test app (like cpu throttling test from playstore) to see how the phone reacts when it’s heating up.
You can see the temperature of each cpu core. Should the phone also start to lag after some time or crash it may be due to overheating.
Look at this (a recent core module teardown)
The pink square is a thermal pad (on top of ram&cpu) used to connect semiconductor components to some kind of heat sink. In this case just a metal cover.
Some are self-adhesive, but not all. It may happen after time that the heat conduct decreases and the cooling becomes insufficient leading to overheating of the corresponding parts. They can be replaced, but complete disassembling needs to be done.
First of all though you should follow @Volker advice and also check if there are more resource hungry processes running in the background keeping your cpu occupied (also consider network access).