isLowSpeedTransfer
Low-speed when the bootloader did not negotiate a larger MTU, leaving us on the 20-byte packet floor. Valid once connectToDfuMode has established the connection (the MTU is known by then).
Low-speed when the bootloader did not negotiate a larger MTU, leaving us on the 20-byte packet floor. Valid once connectToDfuMode has established the connection (the MTU is known by then).