Sorry, but this is all rather vague and it mixes different topics like converters and round trip latency, as well as products/technologies/drivers: PCI/HDSPe drivers, USB/USB drivers (of which there are already two now).
Finally you also talk about Windows and Apple, where Apple usually has some safety buffers more and where the latency is, according to what I read, a little higher because it doesn't access the recording HW directly like ASIO under Windows does. Maybe only a little difference but there is.
It's also not clear to me whether you compared at the same sample rate (-> converter latency differences) and more important the same (ASIO-) buffer size (which has bigger impact) and here the newer USB driver goes down to 32 samples ASIO buffersize (the old USB/FW driver up to 48 samples).

Therefore, it is difficult for me to understand the statements you have made and, more importantly, to be able to derive statements as to what latency requirements you really have. What you regard good / bad / required in terms of converter latency and RTL.

Challenging with small cases. You know heat problematic and to get a sustained high CPU clock without throttling.
This is required to be able to work with lower (ASIO-) buffersizes.

I had a Lenovo Business Laptop around 6y ago and connected my UFX to it at that time to check simply music playback with smallest ASIO buffer sizes of (at that time 48 samples). It failed, audio drops that needed an ASIO buffersize of at least 256 to get entirely rid of it. By this you have a RTL of only ~13ms. Not so nice.
Important there was that the laptop had a dedicated GPU chip.
The design to have a GPU inside of the CPU was not so beneficial as at times it seems to block I/O and what not.
Audio outages went away when e.g. running Firefox through the dedicated GPU. So, good that it was there.
What I want to say, you can have good-luck and bad-luck with a laptop same as with Desktops.
But if it runs then the latency values are the same.
The only problem is, that you won't be able to run the same big projects like with a performant desktop there will be tradeoffs and in a laptop the BIOS does not allow you for thermal reasons to deactivate energy saving to get lowest DPCs which is important if you want to work with lowest (ASIO-) buffersizes and thus latency.

But generally, an UFX+ with USB3 or Thunderbolt should give you the same low latency.
You only need to look to get a balanced notebook with not too high DPC latencies.
One of the newer Apples with thunderbolt and a little bit higher cases might be beneficial.

In regards to minimum system requirements see manual.

To get a good desktop/laptop which is suited for near real-time audio processing I would definitively contact one of the builders who specialiced on offering well suited PCs/Laptops for this purpose.
And then try to get maybe even an TB based system like the UFX+ which has the lowest RTL. and Thunderbolt is external PCIe, although the RTL values of the other drivers are not far from that ... see my table.

In terms of RTL (round trip latencies) you can see from this table from my blog, that there is really not much of a difference between the different RME products / drivers ..

The new MADIface USB driver allows you to choose ASIO buffersizes down to 32 sampes, compareble with Thunderbold and PCIe drivers.

See this blog article: https://www.tonstudio-forum.de/blog/ind … 8-RME-UFX/

Here the overview: https://www.tonstudio-forum.de/index.ph … es-v2-jpg/