Having some problems with concurrent COPY commands

I've been doing some design investigation and ran into an interesting snag I didn't expect to find on 9.4 (and earlier). I wrote a quick python script to fork multiple simultaneous COPY commands to several separate tables and found that performance apparently degrades based on how many COPY commands are running.

For instance, in the logs with one COPY, I see about one second to import 100k rows. At two processes, it's 2 seconds. At four processes, it's 4 seconds. This is for each process. Thus loading 400k rows takes 16 seconds cumulatively. To me, it looked like some kind of locking issue, but pg_locks showed no waits during the load. In trying to figure this out, I ran across this discussion:

Which came after this:

It would appear I'm running into whatever issue the xloginsert_slots patch tried to address, but not much discussion exists afterwards. It's like the patch just kinda vanished into the ether even though it (apparently) massively improves PG's ability to scale data import.

I should note that setting wal_level to minimal, or doing the load on unlogged tables completely resolves this issue. However, those are not acceptable settings in a production environment. Is there any other way to get normal parallel COPY performance, or is that just currently impossible?

--