>> Maybe this would be an improvement.  But sorry, I can't help in testing:
>> while slow `M-x locate' used to be a problem in the past, nowadays it
>> finishes within 1 sec, so I simply won't notice the difference :-)
>
> FWIW, here it takes much longer than that.  It would be very welcome if
> someone would want to work on making it asynchronous.

What I can do to help is to post my current customization: