Rain filling cauldrons is not affected by randomTickSpeed

If you bump up the randomTickSpeed to a high number, it does not speed up the process of cauldrons being filled by rain.

Expected:
Rain is on, a grid of cauldrons been placed, not snowing, not a desert.
randomTickSpeed = 3: slow filling.
randomTickSpeed = 3000: fast filling

What happened:
randomTickSpeed does not affect it at all

it would make sense cause it randomly fills the cauldrons the way crops grow.