You need to differentiate between two latencies.
RTL over USB/FW/...
System internal latencies (DPC) cause by drivers blocking CPU cores where audio related task need to wait, until the CPU becomes free.

Sure, the more channels need to be transferred the more bandwidth you use over USB/FW and the more audio data need to be transferred in time / in the same time interval.

If you want to work with smaller ASIO buffer sizes or even higher sample rates then you put even more stress to the system because the amount of audio needs to be transferred in smaller portions and with higher sample rates the amount of audio data becomes bigger per time interval. CPU load and interrupt load will increase.

> A Madiface USB (128 channels) would probably require more system overhead that a Fireface 800 (56 channels)
> and therefore might have worse latency performance on the same computer. (
> Setting aside any differences in data handling between USB and Firewire for now.)

As long as your system is powerful enough (CPU, ..), uses sane performance settings and has no bad drivers installed then your system will be able to handle a FF800 or even an HDSPe MADI FX with 3 MADI busses.

The latency over USB / FW / PCIe (RTL - round trip latency) only depends mainly on sample rate and ASIO buffersize.
The RTL with RME drivers is very low in general, because of the good hardware and drivers.

Sure its a difference whether you have a 2ch, 30ch or 128ch audio interface. But with a not too old system its possible to transfer so many channels. The latency (RTL!) is independend of the number of channels.
But you should take care that your computer is setup well and has good drivers installed, so that the DPC latency of the computer / of Windows does not become too high, because this can block CPU cores to process audio in time. This you can compensate with higher ASIO buffer sizes up to a certain point where your system might be too inefficient.

Why some companies still offer turnkey systems for audio where a good combination of HW and drivers is preselected and well tested.

Awesome, thanks Ramses very helpful. I'm currently using 3 firefaces because I got the second two very cheaply with hopes of being able to use them as one interface in Mac OSX only to discover that I still have to use an "aggregate device" which significantly impacted performance at 64ms in Ableton Live (which works great with just 1 Fireface). So I was considering getting a MADIFace and ADI 648 and using the 3 firefaces for A/D/A conversion via ADAT through them (and eventually swapping the madiface out for a UFX+ at some point).
I'm currently running an old 2012 Mac Mini when they still had the quad core i7 in them which has been performing just fine with one FF800. Maybe I'll just see how an aggregate device works works when I get a new Macbook Pro before fooling with MADI.

Yeah I won't get an M1 Macbook Pro for my DAW, I'll buy a used Intel 2020 version probably. I'll wait until an M2 is out and everybody has rewritten their code in and then buy one used in about 4 years hahaha. I'm a very lagging adopter for my DAWs for this exact reason. I change my DAW computing setup no more than about once every 6 years to avoid these hassles.