Feel free to discuss any harware topic here
I'm sure Andrew will reply to your post next time he logs on. In the mean time let me contribute what I know.
I am currently in the process of evaluating / testing the current version of the Datapak Gadget (the v1.1 in the 4th video), not from a technical prospective more from a users point of view. It is currently configured so the flash memory on the Gadget represents a 32k RAMpack. You can save or store your data to that like any other RAMpack, so to answer your question "how fast is it?" - it's as fast as any other RAM on the Organiser. Once you have saved to the on board RAM you use the Gadget operating instructions, screen and small key buttons to transfer data / files to and from the SDCard.
The Gadget is currently back with Andrew being fitted to an LZ. When I next have it, I'll be able to test some on my utilities and following on from your question I'll do some read write speed comparisons between Datapak, RAM A: and Gadget. I don't have a Flash Datapak so can't do that comparison (unless Peter wants to lone me one).
I'll publish my user review when I've done a full evaluation, but suffice too say with the software development by by Lostgallifreyan the technical reproduction by MartinP and hardware additions by Olivier and Andrew the Organiser has never been so well presented since the 1980's
I'm not sure if you are talking about the top slot interface or the datapack gadget.
The hardware interface has a ROM interface and hardware IO ports.
If you are asking about the speed of the ROM interface then it is as fast as a RAM pack, yes, as it has to keep up with the slot interface. The code runs on two cores and handles the organiser signals in firmware. The RP2040 has hardware PIOs but they weren't much use with the serial addressing of these packs. Fortunately the RP2040 is clocked at a fast enough speed (without overclocking, which some people have done) to handle the interface.
If you are talking about the 8 bit input and output hardware ports on the card then again, the answer is yes, they run at the speed of a RAM pack too. As the ports are attached using some unused states of the slot interface that the datapacks use (standard Psion interfacing) the accesses to the ports are handled by the same code that handles the datapack interface. The reads and writes are done in the code that would have performed the unused state actions in the datapack gadget code. To access the ports I have written some 6303 code that accesses the port using the special states. That is inserted in the main menu by the ROM code on the PCB.
That datapack gadget uses the same code and hardware as the top slot PCB. It's just shrunk down. It also runs at the full slot speed so yes, same speed as RAM pack.
As these bits of hardware are running flat out handling the interface they can't do much else so when handling the menu they don't respond on the slot interface. To emulate a datapack you have t exit the menu.