iPhone 5S runs very slow

Hi,
I have an iPhone 5S that suddenly started to run very slow (for example when I hit the Camera app, it takes 10 seconds to show actual live preview). The boot time is also a few times longer than normal. I am running latest iOS 10, I will not risk updating to iOS 11, as it seems the phone is malfunctioning and there is in principle a risk of ending up with error -1.

However:

- If I disconnect and reconnect the battery those effects seem to vanish. Camera app starts to work instantly. But after a day or so the phone is sluggish again
- I noticed that there was some dirt in the front panel assembly connectors (of course to clear them I had to disconnect the battery). So in the end I do not know if it was disconnecting the battery that fixed the problem temporarily or diconnecting battery. Some dirt might have remained as I did not have access to the air compressor at the time.

MY question is: what further steps I can take to troubleshoot the Phone? The admittance fee to the AASP makes contacting Apple not economical, and in my country there is no Apple Store nor a Genius Bar.