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.