Well.. that works for AC.

A 4 car on DC is much quicker than an 8 or 12. From driving experience.

Always prefer a 4 car to drive. Quicker off the mark and less DOO cameras to observe

I think your experience may be due to current capping while on DC, and aligns with below.

When you multiple 377s, the software counts the number of motor coaches and divides the current limit between that number. I have got the values somewhere, not dug them out, so this is off the top of my head :

I /think/ it is something like a single 3car 377 is 1500 A and a single 4car 2250 A i.e. 750 A per pack.

But when you multiple them, it is not simple addition of 750s. 2 x 3car is not 2 x 1500 A but something less, and also 2 x 4car also not 2 x 2250 A, then when you add further units, it is even further less than the sum of the individual units.

It has to work like this - the nominal country current limit is 4500 A (yes ? Nick ?) if you just banged together 4 x 3car you'd get 4 x 1500 which is 6000 [more than the country limits], and 3 x 4car 6750 (which is just below the absolute 6800 A limit).

Also, there are two ways to make 12 cars - 3x4 has 9 motor coaches and 4x3 has 8 motor coaches - so the software equalises current cross the complete train to equalise timetable performance.

I may not have the Ampere figures all precise, but that is the gist of it. At least that was how it was explained to me when 375s and 377s were new.

Anyway, it is one possible answer to why a 4car 377 on DC appears to perform better than a bigger formation.

I think this principle did not apply to any of the Desiros, be they 444 or 450, when new, but in more recent years there has been a mod to for current capping but unsure what it is. There has to be something as nominally the traction pack on a 444 is the same as a 450 - and a 12car 450 has 12 but a 10car 444 8.

700s are different yet again - while these do not multiple with each other in normal circumstsances - but run solo, they too have current capping as solo units that favours 8car over 12car. GSM detects where the train is. A FLU is 6800 5000 and 4000 A for inner suburban outer suburban and country area, RLU 4500 3750 3000 A. You can see here that compared to train length, the current capping favours the shorter train.

Zero idea how all this will work with 701s.