Back in June, it was discovered that OS X didn't scale the TCP window size beyond 64KB when copying files over an AFP share. This artificially limited the speed that users could transfer at. Now that dynamic scaling is in place we see some major improvements.
That's more like it! When transferring files over the AFP protocol, it looks like OS X now performs more-or-less identically to Windows 8. Performance is well over double where it was in 10.8.4, which bodes well not just for the 2013 MacBook Air but for any other 802.11ac Macs that come out in the future. Transfer speeds over SMB haven't changed, unfortunately, but we wouldn't read too much into that—Apple is due to replace the Mountain Lion SMB implementation with a new SMB2-based protocol in Mavericks. While we haven't been able to test the latest OS X 10.9 developer betas, we're confident that Apple will have ironed out any issues by release, since the company is clearly aware of the problem.
Notably, both devices will need to be updated to achieve the improved file transfer speeds.