Why did the DC-3 have a much higher cruise speed than the HP.42, despite their similar weight and power? A review of the classic DC-3 and the Handley Page HP.42 yielded a surprisingly close match in empty and max gross take off weight at around 18,000 and 28,000 lbs for the HP.42 and 16,000 lbs and 25,000 for the DC-3.  The HP.42 cruised at 87 knots on around 4x500 hp, and the DC-3 at 180 knots on 2x1200 hp.  The HP.42 was a lower aspect biplane, the DC-3 a graceful mono plane, the epitome of aviation advances for that era but...

Questions:  How much of the DC-3's speed increase was the result of improved engine/prop efficiency?  How much from the DC-3's speed increase came from its beautiful Prandtl inspired wing?  How much from retracting its gear?  How much closer would the HP be with those improvements, while remaining a biplane (perhaps with improved struts)?

I do not expect an exact answer, I am more interested to see how the numbers were crunched and a short explanation why.  Thanks.
Struts every which way, uncowled engines, wheels with fenders hanging out, wires, bits sticking out here, bits sticking out there, corrugated skins, fabric wings and tail... it's a mess; a flying Truss Bridge.  There's just too much to try to quantify it like that.

You could probably take a DC-3 and start adding all those things one at a time, and each item would knock off 10-20 kt, and eventually you'd end up down at 87 kt, but the slower you get the smaller each accumulative penalty would be.