Apple M1 Pro Max + Computrainer

Running on M1 Pro Max,
all is good, until I connect Computrainer via USB-C.

It detects Computrainer but FPS drops to something like 0.3 fps. As soon as I disconnect fps is good again.
It seems like serial communications somehow interrupts playback making it unusable.
It looks like scheduling bug between serial I/O polling and video render ?

Activity monitor shows that Zwift is running in Intel mode on M1.