With some Yamaha MIDI devices (for example the DD-65 drum machine) the iTwo recognizes one only note every one or more seconds. I have read that it is a problem of clock setting, internal or external, but not all Yamaha devices have the settings for the clock. The only solution I've found is to add another MIDI adapter (M-Audio 1x1) and use iTwo as only audio device (or with other MIDI devices...).
The problem appear to be "hardware" (firmware configuration?) and it is present with any system: Windows 10, MacOS (MacBook Pro) and iOS (iPad / iPhone).
Firmware and drivers (Windows) are updated to the latest version.