Any time a block adjacent to snad updates – and this includes e.g. cactus advancing a growth state right on top of the snad bock – it triggers a growth update, leading to cactus growing its second block near-instantly after placed or trimmed:

This screenshot was taken while the game ran with /gamerule randomTickRate 0, which should mean that nothing grows unless it gets bone-mealed. However, the cactus just kept growing, I'd say roughly every 16 ticks, since that's the number of growth states cactus has.

Snad version: 1.18.2-1.22.04.15a (Growth Speed = 2)
Forge version: 40.1.54
Mod pack: The Phoenix's Magics and Goodies 1.4