The Meaning of Life, the Universe, and Everything.
For a 10x speed-up:
gamerule randomTickSpeed 30
According to the wiki, it will not really "speed-up" time, but it will select more blocks to update at each game tick (here 30 blocks instead of 3). Aggressive values of randomTickSpeed seems to have unwanted side effects/create race conditions with BUD devices, since many blocks are updated at each game tick.