Video issues FPS dropping with lenght of recording.

Daphone

My Z phone have done something to itself, when i do a video recording ( 1080 / 30 ) the longer the recording go on the fewer the FPS become, so after 2 - 3 minutes of recording the average FPS on the recording are 12 FPS.
I have not tried anything new on the phone like apps or what ever, so pretty strange.
I assume a factory reset could do the trick but really i cant be bothered with that, it is after all rare i use the camera for video, and photos i only snap like 1 every month on average.

So i will just let it be, and hope i can rob a little old lady of some money to get a new phone some time next spring, lost of interesting things going on in MID and high low end phones.