[Q] Major difference in 3G data speeds between ROMs topic

I just discovered a major difference between my 3G data speeds between ROMs on my HTC One

I've done a lot of tests to prove my point, I've been switching between stock Three UK ROM/firmware (Android 4.3) and ARHD (Android 4.3-4, US Dev / international firmware) for 7 consecutive days, and every day performed 5 "control" speed test, on each ROM/Firmware, in same location.

1) on Stock Three UK ROM/Firmware I'm getting on average 6Mbps (max 8Mbps, min 1.5 Mbps)
2) on any others I'm getting on avg. 2-3Mbps (max 5Mbps, min 1Mbps)

So my question is, why stock branded ROM/firmware is on average 3Mbps faster than any other ROM/firmware configuration,
and is there any way to improve speeds on non branded rom?

*I think that's because only stock is actually using DC-HSDPA, is it possible to force enabling of it on other roms?