I looked in the LS-40/LS-4000 manual and the LS-40 is USB 1.1. However, the rated scan times for the LS-40 are only a little slower than the LS-4000 over Firewire, per the manual. Using ICE slows down the rated time by more than 2x for either scanner. I don't know which part of the system is the limiting step. I have a LS-40 but it is packed up at the moment - my memory is that it wasn't obviously slower than using the LS-30 over SCSI.
Although off topic for this thread, several versions of the Minolta Dimage Scan Dual used USB 2.
I was talking this over with a friend earlier today in general terms.
I don't know exactly what the mechanism of data transfer is for scanners, but truth be told even at 4000ppi the acquisition time is short enough that you probably don't need super fast transfers. On paper USB 2.0 is faster than FW400(480mbit/s vs 400mbit for FW400) but in practice FW often gets higher sustained speeds.
50 pin SCSI can manage 40mbit/s in theory(although IIRC in practice it's really sensitive to things like cable length. Some claim termination isn't necessary in LVDS implementations, which IIRC the Coolscan III is, but I've also seen slow downs on internal hard drives without terminating properly). USB 1.1 is 12mbit/s. For some perverse reason, or actually I have my theories as to why, Apple made USB booting relatively easy in a lot of USB 1.1 systems, but made the process more complicated and less intuitive when they started shipping USB 2.0. I once booted a Titanium Powerbook to 10.5.8 by a USB external HDD-it took 15 minutes to reach a useble desktop, where the same computer will get there within a minute on the internal ATA/100 bus.
All of that aside, what I'm really saying is that I don't know how much of a difference the bus speed makes in practice for a scanner.
Some of the fastest film scanners I've personally used have been SCSI, but they are also some of the oldest. It's been probably 5 years since I used my Coolscan III(I'm only just starting to get some of this stuff out of storage after relocating two states away in 2020) but I seem to recall it actually being pretty zippy. With that said, as best as I remember, its max resolution is 3200dpi. I suspect if you turned a IV/V/4000/5000 down this low, they'd probably be as fast if not faster, but again I've not tried it. I have a Polaroid Sprintscan 35 in storage somewhere and I remember it being really fast, but it's something like 2400dpi if even that fast.
I do have my original Coolscan here, and I'm curious to try it, but between the resolution, the lack of ICE, and that it only takes mounted slides I'm not in a big hurry to use it. Maybe I'll try the next time I have one of my PM G4s out. The only other SCSI scanner i have ready access to is a Leafscan 35, which I'd love to use but I've never been able to sucessfully get it to complete calibration. To be honest, if I ever were to get that going, I'd probably forget going the old Mac route just for convenience. I already have an National Instruments GPIB USB adapter on my 2019 iMac that's my main desktop-it interfaces to a couple of HP Multimeters and I do data logging from those in a Windows VM. It SHOULD be easy to add the Leafscan to the GPIB bus and run it that way, but kind of no point too unless it's actually working. GPIB is slow compared to SCSI, but also somewhat more robust(IME-I still run some GPIB at work, or rather HPIB since it's all HP stuff and I'm using HP interface cards and software, and it honestly just works).
The next time I set my scanners up, though(I don't have a permanent home for them-it's mostly just claiming the dining room table, and usually when my wife's at work
) I'm going to try something. I typically use one of the old Bondi Blue Apple USB keyboards, the type that shipped with the first iMac G3, with the Mac Pro. All but the very last Apple hardwired keyboards had USB 1.1 hubs. Of course that's zero issue if you're plugging a mouse into them, which is the main reason why the boards have USB hubs. Still, though, even a USB 3.0 peripheral plugged into one will run at 1.1 speeds. If my Coolscan V is measurably slower running through the keyboard hub compared to directly into a 2.0 port on the computer, we'll know the interface makes a difference.
I'm not sure anything can be as slow as the first flatbed I remember my dad buying in the mid to late 90s, which ran off a printer port. He had a ZIP drive that connected that way too. Those were painful...