The problem is high IO, mainly read 50MB/s (100%), for about one minute of despair, lowering the quality of the android realm user experience.
The only way to know for sure, would be https://www.xda-developers.com/check-io-usage-iotop-android/amp/
So, i would need root and a specially compiled kernel (rom?).
But, if you could hint me, mainly if yours have an iotop like app, it could be enough.
I tried stopping gallery, i thought it was scanning media, didnt help.
During high IO, i saw zygote use a bit cpu but on top of the list, but that is a proccess, not an app (that we could stop).
I use simple system monitor app.
And i need to know HINTs of the app/proccess, not the overall IO neither what specific proccess here is causing trouble (as hints may suffice), so I can try stopping them by chance.
Thats why my question differs from: View disk utilization / activity (also the accepted answer there doesnt answer about showing specific proccess IO, that is in the body of the question, therefore the accepted is just a partial answer).