Are you measuring this against wall-clock (i.e. duration) time, or just going by what you see on screen?
For example, does any given download take, say, 5 minutes on the Windows machine, and 50 minutes on the Mac (which is what your statement implies)? Or do they take about the same amount of time.
The reason I ask is that for the longest time (and maybe still), Windows reports downloads in kiloBITS per second whereas Mac OS apps typically report kiloBYTES per second. Since there are 8 bits per byte the Windows figures are inflated. Measuring against real-world time (using a clock, watch or other timer) is a far better way to gauge relative download speeds (and should be tested against a decent-sized download, not just a 10KB file).
I know BYTE=8 bits !
I converted download speed at windows to kiloBYTES/Second
at my PC : 150 kiloBYTES/Second
at my Mac : 12 kiloBYTES/Second
I download same file at Win 5 Min but at Mac maybe 1 houre