I'd prefer a smarter solution for this that automatically disables updates depending on which type of property was changed, but for now, this will significantly improve the performance of cactus farms.
The newly placed cactus block at the top cannot have updates disabled, though, since it needs to check its surroundings in case it grew into a space with a solid block next to it.
Thanks @KingOfTurkey38 for bringing this to light.
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