Does the speed vary at different (i.e. busy) times of the day? I'm thinking the answer is probably 'yes'.
Reason I ask is that I'm supposed to have a 4G connection, but there are no 4G mast in the immediate vicinity, and 3 / 3.5G is abysmal at busy times. Therefore I am thinking of getting (or making) a directional 4G antenna to try & pick up a 4G signal from a more distant mast. But if that slows to a snail's pace to it won't be worth the effort.