Hi Frans,
I did try connecting my p121 as controller to my sx 900 via iPad and Midiflow app.

iPad connected to p121 via lightning adapter, sx900 via bt 01.

I think it felt like there was just the very slightest tinge of latency when using p121 to play piano sounds on sx900, which I don’t get when I use my laptop as interface, and just plug in the 2 USB cables.

The laptop set up just felt a bit more responsive, if that makes any sense.