Why is the mouse behaviour different when Assistive Touch is enabled?

iPadOS 13.4 radically changed the way external BT mice behave in the OS for some reason. I now realise that the mouse is much smoother when connected but when Assistive Touch is disabled. However, in this mode the extra buttons on the mouse don’t do anything.
When I activate Assistive Touch, the mouse lags weirdly and goes all kinds of directions making it much less smooth and useful, but the extra buttons work.
Have you guys figured out how to have both?
Thanks,
MM
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