I will have to see if their NFSv4 implementation has improved any. When I added support for EFS to my company's NFSv4 client we ran into a couple of performance bottlenecks and just general spec non-conformance.

Specifically, we noticed lack of support for these features:

- session trunking (and, in general, multiple channels)

- multiple concurrent requests on a channel (ala ca_maxrequests)

- callbacks

ca_maxoperations was quite low (10, I think?), and they limit the number of parallel clients that you can access EFS with at once.

What this amounts to is that you can reach acceptable performance on file reads/writes, but metadata heavy access patterns have no hope of reaching the advertised IOPs. It's a shame because frankly metadata performance is something NFSv4 excels at.