I have also found 2.4ghz to be the culprit of low speed and this is in a relative large 4 bedroom house and being in the same room as the router. The signal around the house shows as fine (min of 4 out of 5 bars) so it's not bad signal.
I am not living there at the moment, so assumed it could be a problem with Virgin Media's network as whenever I went there I was lucky to get 60mb/sec through my iphone, and moving around house it could drop to 20mb/sec. As soon as I restarted the router it would get a higher speed (but still no where near max) so for along time I put it down to Virgin network and thought I'd deal with it when I move in.
Couple of days ago I was having issues with my CCTV so decided to finally have a look.
I turned off all the auto channel steering functionality as apparently this doesn't work well on VM. Tried changing 20/40mhz on 2.4ghz but as soon as the router restarted it would return to 20mhz. Tried changing channels between the usual 3 options 1,6,11 to no avail, might get a little more speed but not much above 80mb/sec.
So I separated out the 2.4ghz and 5ghz networks (would rather not have to do this) and problem solved. On 5ghz it's getting the full 200-220mb the connection supports. Even in the furthest away from the router still gives well over 100mb/sec. In fact, on 5ghz I could still get over 60mb/sec at the bottom of the garden or drive at either ends.