We gave it another shot and upgraded to 7.0.16, however the problem persisted.
In our case, the issue was that the new dynamic ticket algorithm provided far too few tickets:
(cf. mongo/src/mongo/db/catalog/README.md at master · mongodb/mongo · GitHub )
After the upgrade to 7.0.16 the available read/write tickets drop as low as 8 tickets in comparison to 128 tickets in 6.0.13
We managed to resolve the problem by setting storageEngineConcurrentWriteTransactions and storageEngineConcurrentReadTransactions to a fixed value of 128, just as in MongoDB 6. Afterwards the performance also returned to MongoDB 6 levels!
If you’re facing a similar issue, this might be worth trying!