As Bill says, as of now the LEDE tree doesn't have SMP enabled due to a bug with dual ethernet interfaces (WAN & LAN). If you aren't using the WAN ethernet port (ie using the internal VDSL modem) then you can run the SMP kernel (you need to revert a commit and build the image, or get an image from someone which has it reverted, or built out of Felix's staging tree). I am running this and it works well, as I am using the internal VDSL modem, ethernet and wifi 2.4/5GHz capabilities, but not the WAN port.
Without SMP you won't be able to saturate 80 / 20 VDSL over 2.4GHz / 5GHz wifi - that will chew though all the CPU.
With SMP you will have some CPU spare, but flat-out 5GHz performance is still down on stock firmware (I see 180Mbit vs over 300Mbit with stock).
@ejs is right I believe, I don't think either of those functions are in LEDE at the moment which no doubt contributes to the performance differences. @ejs - do you know where I can get the sources of those utils? Am interested to see what makes them tick.
Still, even without it makes a very nice and cheap combined dual band VDSL N/AC wifi router with a reasonable amount of RAM and Flash, and breaks the shackles of operator supplied firmware. I last picked up a couple of HH5A's for £15 each, which is a complete bargain in the scheme of things.
Below are a couple of tests I've run. Nice to be able to run TBB ping monitor on this box at last