Since I'm in a ranting mode -- here's a good example: you're limited to _ONE_ IO per shard in the history service:

https://github.com/temporalio/temporal/blob/e22e6304b3c4a409...

https://github.com/temporalio/temporal/blob/e22e6304b3c4a409...

Temporal does a crazy amount of database operations and all of these are behind that mutex.

Oh, and you can't change the shard count on existing clusters.

Great stuff.