Yes, when you see
mmcqd on the top-3 processes most of the time you can assure there is a bottleneck created on any storage/flash. You can check that fact easily at the developer options in Android (how to get it), by enabling view CPU usage, the blue color represents I/O operations (more info).
My scenario was more general than the one of the original question: the tablet was already cleared factory-reset, no new apps at-all except chrome and netflix, even re-flashed the stock image but it still was desperately laggy (FYI - Nexus 7 model 2012 with 5.1.1). This is specially true when connected to WiFi or CellData, so before proceeding it's better to enable
plane mode for a faster operation.
I found the following fixes to be sufficient:
- [optional but recommended] IF you are root OR have custom
recovery (any), you can manually
trim the storage for a boost of
performance (how to). This is specially true since the last
time it was used years ago apparently worked well but apps or reflash might have filled space without a chance to trim (more info).
- [optional but recommended] The newer versions of the Google Play
store include more services and functions (even API and sync), which I found
specially heavy for those older flash storage. Uninstall unused new Google Play items (such as music, books, news, films) and then go ahead to the sync
menu and disable most of the items (how to). You will surely
find there an item in error, but anyway it's better to clear most of them.
All these are background I/O (instead of foreground I/O) lagging
your device as soon as you connect to network (WiFi/cell).
- [optional but recommended] On the above mentioned developer options menu, scroll down at the bottom to find
limit background process and select
no process for a radical approach. That will eliminate background apps I/O adding up.
These general actions totally solved the I/O issue of the old Android (constant mmcqd). However obviously it is not perfect and there might still be other additional problems, because still modern apps take a small lag to react (for CPU and disk read), and first-boot-up of the system needs some patience (all this is expected). Newer apps and google services do expect newer devices speed.