samsung – How to determine which app or service is frequently writing to storage and causing lagging?

When I turn on the screen on a sleeping (but not powered off) high-end Samsung Android 7 Nougat device, the device is often very unresponsive (lagging) for 20-200 seconds.

Using the excellent Simple System Monitor (SSM), I can see that it’s not a CPU or RAM issue. There also is only minimal network activity during this time. The issue appears to be something writing to internal storage. In all the SSM graphs, it’s the only graph that shows a high level of activity during the unresponsive period.

How can I determine which process is performing all that writing to internal storage ? The device is not rooted, nor does it have external storage.