The intention is to gain access to the USB port in regular designs for GPIO and other needs. It seemed so sad to not be able to use the USB port in this way…
The resulting port is only a USB 1.0 Full Speed port, if I understand correctly, so 1.5MB/s is the theoretical limit. I have yet to push it really hard, but the bootloader (upon which the serial code is based) is pretty quick and reports 145kB/s - and it has other stuff to do beyond just transferring.
One other thing to know, is that it has been working in other weird clock setups. For example, one of my examples seems to run well when I set the (solitary) PLL to 96 or 192MHz and just divider the clock down to 48MHz. This gives some flexibility to do data acquisition at the higher frequency, as long as you’re quick.
@lawrie.griffiths, the original adapter of the bootloader to the USB SERIAL context may have other thoughts too.