>Only one thread actually mutates anything, and its the same one over and over
Sounds great in theory and usually results in fantastic average case, but you get a hot partition and suddenly you can't work share and things go south. It's not a perfect solution.
If you have a single serialized+synchronous business context in which activities must be processed, you literally have no other option than to use a single physical core if you want to go fast.
The trick is to minimize the raw amount of bytes that must enter into that synchronous context. Maybe the account disclosure PDFs can be referred to by some GUID token in AWS S3, but the actual decimal account balance/transaction facts should be included in the event data stream.
One other option is to talk to the business and see if you can break their 1 gigantic synchronous context into multiple smaller ones that can progress independently.
Sounds great in theory and usually results in fantastic average case, but you get a hot partition and suddenly you can't work share and things go south. It's not a perfect solution.