I believe there are reasons why e.g. io_uring could be inherently slower in some cases, and I tried to point some of those out.

With io_uring everything happens in the backend process, and so consumes some of the CPU time that might otherwise be spent executing the query. All the checksum verification, memcpy into shared buffers, etc. happen in the backend. And those things can be quite expensive. With worker this happens in the other processes, spreading the overhead.

Of course, on truly I/O-bound workload (actually waiting on the I/O), this may not be a huge difference. For warmed-up cases it may be more significant.