The more you weigh, the more power required to go atget to, and maintain a certain speed.. but at the same time, generally, the more power you can produce. On the flat you're mostly working against air resistance, a larger framed rider (less aerodynamic) will need more power to overcome the resistance than a smaller rider. Generally aerodynamics of rider & weight of rider will have a strong correlation.
30mph is not at all slow, if you were able to maintain that for 20 km you'd actually post reasonably competitive times in a pro TT!
A better groupset might save weight (although that's more important with climbing & accelerating), it might also give you biggera better selection of gears to& better power along withtransfer. A carbon bike might be more aerodynamic.