Multiple threads in a single process doesn't preclude data duplication in any way; and optimal IPC is always going to be slower than optimal in-process message passing. I wouldn't consider bringing a database into the communication channel to ever approach optimality.

As to GC, that presumes that you are using GC. For a request/response style server application, there are more optimal techniques than GC: make sure all request-associated allocations come out of a per-request heap, and you can free that heap en masse, in one go, once that request is done with.