Official comment
Currently, as users can select 3 kinds of effects for MultiFx, multiple effects with different minimum time can be selected at the same time, eg. ROLL @ 10ms and ECHO @ 1ms,
we set a common minimum time at 10ms.
As users may switch in between MultiFx and SingleFx, we need to set the same common minimum time at 10ms. It would be very difficult to have two different minimum time for all the MultiFx although our engineers have investigated the ability to change this for a SingleFx.
Regarding High Sierra, it is currently unsupported and any issues found may not be addressed until the OS's final (public) release.
Thanks for your feedback.