In its extensive review of the new 2013 MacBook Air, AnandTech notes an issue with the machine’s new 802.11ac WiFi capabilities that it says is limiting the faster Wi-Fi chip’s potential. While it was able to get an average of 533Mbps using the iPerf networking tool, Anand found real world file transfers would only get 21.2MB/s or 169.6Mbps:

I disabled all other wireless in my office. Still, no difference. I switched ethernet cables, I tried different Macs, I tried copying from a PC, I even tried copying smaller files – none of these changes did anything. At most, I only saw 21.2MB/s over 802.11ac. I double checked my iPerf data. 533Mbps. Something weird was going on. I plugged in Apple’s Thunderbolt Gigabit Ethernet adaptor and saw 906Mbps, clearly the source and the MacBook Air were both capable of high speed transfers. What I tried next gave me some insight into what was going on. I setup web and FTP servers on the MacBook Air and transferred files that way. I didn’t get 533Mbps, but I broke 300Mbps. For some reason, copying over AFP or SMB shares was limited to much lower performance. This was a protocol issue.

According to the review, the problem is likely with the OS X networking stack that is for some reason artificially limiting the capabilities of 802.11ac:

TCP window size defines the max amount of data that can be in flight before an acknowledgement has to be sent/received. Modern TCP implementations support dynamic scaling of the TCP window in order to optimize for higher bandwidth interfaces… OS X didn’t scale the TCP window size beyond 64KB, which limits performance to a bit above what I could get over 5GHz 802.11n on the MacBook Air. Interestingly enough you can get better performance over HTTP or FTP, but in none of the cases would OS X scale TCP window size to 256KB – thus artificially limiting 802.11ac.

Unfortunately, AnandTech wasn’t able to find a workaround for the problem and later reports from others confirmed he isn’t the only one experiencing the issue. It’s also not something that appears to be fixed with the current developer build of OS X Mavericks. The good news is Apple will likely be able to fix the issue for early adopters of the new MacBook Air in a future software update.

Further analysis from ArsTechnica found that 802.11ac, unlike on OS X, was functioning properly over Windows on Boot Camp with transfer speeds around 218 percent faster than on OS X.