If I understand correctly, there are two different limits: per IP and global. You can see the performance chart at:
http://stats.metabrainz.org/dashboard/db/musicbrainz-web-servers-status
Switch to “This month” view and hit the refresh icon in the top right corner. You will see that performance has dramatically improved compared to April and early May. I assume (I am not part of the dev team) that upgrade to new Postgres and/or changes to schema and/or some other patches have helped to improve performance.
And while I am very happy to see this improvement and grateful for all the efforts that lead to it, I must say that 1 request per second reminds me of my youth, which, sadly, finished before this century. This rate is a bit slow even for an individual consumer, and, obviously, it is a non-starter as a real-time data provider for any decent-size project.
It would be very helpful if someone from the development team explains the bottlenecks. If it is possible to create a JSON API mirror, I will consider hosting it.