Short story: Samsung S3 running stock Android 4.3. Was sluggish, turned on "show CPU usage" in developer options and it hovers around 2/2/2.5. Stopping all active apps doesn't help. I see "mmcqd", multiple "kworker" instances, "system_server", "systemui", "surfacefinger" etc. in the related list of processes.
Didn't know how normal this kind of load is for an Android device, so I checked on an LG 4x HD (Android 4.0.3) and there I see usage fall below 0.5/0.5/0.5, so there seems to be a problem on S3.
Update: checked a low end Lenovo A Plus and load there is around 12, yet it's quite responsive. No idea what to make of it. In any case, the S3 is very slow and shows a much higher load than the LG.
Sure, S3 has Viber, all kinds of Google sync enabled, but I'd don't see these in the process list and they're not being used (no user interaction with the phone whatsoever).
So, how to find the cause and reduce CPU load to make the phone more snappy?