Went back to the store, same unit. Updated the F/W to the latest version. Same issue (USB MIDI). Need to confess: The problem only happens on Linux (Ubuntu 17.04 this case with the ALSA Virtual MIDI keyboard application). Windows 10 is OK. Switched the same PC to a P6 desktop under Linux and everything was fine, no lag at all. This is the first product I have seen this very notice-able lag under Linux of about a a quarter to half a second. Will need to spend more time with the REV2 to debug this further for example to test with DIN cables and an external USB MIDI interface. Until having figured out the cause and solution, the REV2 will be on hold for consuming pocket money.
P.S: Yes, you will say, oh it's just a Linux user, but well, yes, they do exist and maybe even more surprising, they also use synthesizers.