Hey again @Konstantin,
From what I see from your mongostats, you are queuing a lot.
You have 786 write operation queuing and 128 currently happening. I think your disk is saturating. Too many write operations probably and your disk can’t keep up with the load.
If you have many indexes maybe you could offload the indexes to another SSD by using the wiredTigerDirectoryForIndexes option. You could also offload the logs.
Cheers,
Maxime.