Tomato on RT-AC66U - horrible speeds

I've been trying for some time now to get Tomato working on an RT-AC66U. The problem is wireless speeds. Measuring the rate of an FTP transfer from my desktop machine, I get about 100-200 KB/s (!) in the 2.4 GHz band and about 2-7 MB/s at 5 GHz. The stock ASUS firmware gives a solid 6 MB/s at 2.4 GHz and 21 MB/s at 5 GHz, and my old WNR3500L (running Tomato) gives over 4 MB/s.

I get essentially the same result with Shibby 120 and 138 and a late Toastman flash. I'm using 20 MHz bandwidth...

Tomato on RT-AC66U - horrible speeds