Over DIN, the format is 8-n-1, which results in 10 bits per byte (start bit, 8 bits of data, 1 bit to stop, no parity bits).
It takes just under 1ms to transmit something like a (on DIN) 30 bit Note On message, assuming no other messages are happening on any of the other 16 channels on that cable. Things like system real-time messages, for example, have to be worked into that stream.
Over USB, each individual MIDI 1.0 message ends up as a 32 bit USB packet. In theory, USB could run at the max speed USB runs at, but in reality, most USB MIDI 1.0 devices constrain themselves to the 31250 speed due to other attached devices, etc.
Pete