Apologies for the delay!
Some context about these collections:
1 => Should be relatively small as you shouldn’t have a huge number of collections. How many docs do you have in this collection? Spending that much time in this collection don’t make much sense to me.
2 => Should contain a lot more. This makes a bit more sense.
3 => Tags should also be a very small collection. Few docs I guess? Can you confirm?
But given the fact that a lot of read operations seem to happen in these internal collection, my best bet is that you have a backup plan running every 4 hours?
Do you also have anything else increasing at the same time? Like number of connections? What does mongostats looks like?
Cheers,
Maxime.