I looked at it and thought it looked interesting if you could make use of its software.

A few things I didn't like the look of for performance:

PIC18F (slow with little memory to buffer)
MCP2515 (tiny receive buffer)
ASCII protocol (halves transfer rate)
Throughput tests only at 250Kbps

I ended up using J2534 for my project, but it has some weaknesses:

Easy to snoop
Polling instead of callbacks
Price is higher for a genuine product
Windows registry dependency but can be made to work on other platforms, but no examples with Android
Not usually wireless
Carries other capabilities I don't need hence price

Soon I'm migrating my project to a custom built ESP32 device with a CAN transceiver. Before I even started the project, I made sure I could tweak it so that it could deal with a full CAN bus with minimal latency (<10us the minimum I can measure with what I was using to test - Kvaser Leaf Light HS v2). Out the box, the sample drivers could not. The net effect on large transfers is considerable if your device is slower than your ECU.

From what I've seen of MED17, it transmits every 1ms for large ISO TP groups, and asks in its flow control for 2ms inter-packet spacing to receive but accepts faster. So you might not need the theoretical ~240us (depending on bit stuffing) reply time. Kvaser told me they only do 1ms best over USB.

Thank you both for input, I will look into all this.

USB2 has 125us polling, that's a hard limit you can't do anything about.
USB3 works differently and you can get down to 25us.

Well then USB3 is your best bet.

Yeah, it's probably the best way to go to have all the logic in the MCU and have a higher level protocol for computer comms vs. trying to send every packet from the computer.

I have been digging a bit into J2534 things, and I think I now understand what it offers. I have even localised this:

https://github.com/NikolaKozina/j2534

which is of great help. But, I am still not totally clear on the actual devices. I know the genuine stuff costs $$$, I have also found something called Tactrix Openport2.0 on Chinsese websites. Does this have any chance of working. Or, there is this Toyota Mini VCI cable popping up all the time when googling for J2534, is that anywhere near what I think it could be? Would it work for CAN 500kbs based protocols?

You are a gold mine of knowledge for this You really ought to change your nick to something like CAN-Bus-Guru or something

Why can't it be simple I wonder, it's not a rocket science to have a USB to CAN device. Many standards, proprietary hardware, what not.

Original products are not an option for this use case (not that I am against the principle or anything, quite the contrary), for personal use I already solved my issues with Arduino-CAN, but I was simply thinking about open-sourcing some of my developments (logger/flasher) for ME7.9.10, but I'd need some hardware people could just buy easily and cheaply. For logging ELM is sufficient, hell, I even managed to get ELM to work for flashing, but the complete procedure is 8 minutes instead of 1.5. And it has some really ugly hacks because of the older ELM version (and also most likely Chinese, never cared to check) that I have that does not support some things.

Seems the next step would be to try the cloned OP (it I could only buy it in Europe, Sweden is now hard on imports and duties have to be paid on essentially everything, not a financial problem, but it also equals months of waiting time for delivery, seriously).

How much slower are we talking about for MVCI?