@phooky long shot, but is there a hub involved? When I was building a verilog CDC interface, /dev/ttyACM0 bandwidth through the hub was around 400 KiB/s, while plugged directly into the laptop the FPGA's USB serial port received at nearly 1 MiB/s.
@th no hub, direct connection from Pico to laptop. Linux reports this as a full speed device, so I suspect this is somewhere on the Pico.
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