According to the Halfords website, the crankset/chainset is a PROWHEEL, A1-221C - which has forged aluminium cranks with steel rings and a steel spider (
linky). I seriously doubt that any weight saved by replacing the steel components of the chainset with alloy would make any noticeable difference to cycling speeds - you could probably offset the extra weight by using lighter shoes! Someone else made this point earlier - the bike costs £250 - you're not going to get 'light' for that price - a good deal of people who want light would buy something like a Shimano 105 groupset - which costs half as much again as the whole Zelos bike costs (then they would spend a shed-load on the frame and wheels etc).
My bike is a weighty steel-framed and forked touring bike (probably about 13 or 14 kg) and I usually go out with a hefty chain bike-lock which weighs over 2kg, a tool kit that weighs a kilo and 1.5 litres of water (amongst other things). I can go at about 17 mph comfortably on the flat (I can do more but usually do less) and I often go over 30 mph down hill - if I go on to the drops, there are several places that I go down-hill at 25-30 mph with absolutely no pedalling. Uphill, I slow down to as low as 6 mph. I am not fit and I'm not a fast cyclist. My dynamo lights are always on.
What speeds are you cycling at and what speeds are you expecting?
I agree that a bad fit could make a drastic difference to pedalling efficiency but even with a frame 3cm smaller than the recommended size, I would expect to be able to get a reasonable fit.