Totally pedantic deep dive: the Pico has a USB FS PHY (the numbered versions don't directly correlate to line rate, and FS was defined in 1.0) - the signaling rate is 12Mb/s, but it's impossible to achieve sustained transfers at this data rate under any circumstance:

- USB FS has a 1ms frame limit (HS is 125µs)

- UBS FS Bulk is thusly limited to ~19 transactions x 64 bytes maximum (HS is 512)

- USB FS Isochronous can do 1023 byte transfers, but you can only fit one of those in a 1ms frame (resulting in a giant quantization hole in the packet)

- Focusing on bulk only: the token packet, ACK handshake, inter-packet bus turnaround time minimums, framing bits, CRC bits, and periodic FS SOF packets mean that the actual theoretical maximum data rate is ~81% of the signaling rate

- Bit stuffing optimality issues (required for clock recovery) eat an additional several percent on most data, up to ~17% on pathological data

Therefore: ~9.5Mb/s is the best theoretical data rate that can be obtained with optimal host and device IP and an ideal application layer.

Realistically, ~8Mb/s is the most one can expect on real hardware with an ideal application (and this is optimistically high in my experience.)