ISOstick still contains a controller, its processing speed was 12MB. The tested SD-cards (separate card reader) also had about the same speed, sometimes slightly more.
Following bottlenecks exist, slowest one determines maximum speed:
1) speed of the flash chips
2) speed of the controller in the USB device
3) practical speed of the USB port (yay protocol overhead!) on your machine
4) theoretical speed of the USB port on your machine
5) controller and/or bus your USB port on the machine is attached to.
6) processing power of CPU (routers with USB 2.0 port are infamous for getting below 10MB/s.
7) medium you're likely to transfer your data to (faster or slower than the USB storage device)
8) BIOS init speed of the USB controllers/stacks.
9) memory speed (usually not a bottleneck)
Just consider a PCI USB3.0 card on an ancient Pentium (i80586) and imagine what kind of speeds you'd get there