There are too many "it depends" issues, especially the hardware, and not least the application architecture sitting on top of it.

There's a script in the Message DB codebase that will do some fairly rudimentary read and write performance measures for your Postgres deployment:

https://github.com/message-db/message-db/blob/master/databas...

It's run right on the database, so no network I/O or client implementation in the loop - which is inevitably non-representative of "real world" scenarios.

I got stuck looking for some perf figures too. I realise they are going to vary widely by hardware, batch size, message size etc, but it would be really useful to have a comparison against, for example, a single RabbitMQ instance.

A message store is really not a comparable pattern to a message queue. It's unlikely that the usages would be similar enough to engender a meaningful comparison. They're not as dissimilar as an apples-to-oranges comparison, but maybe an apples-to-applesauce comparison.