Biggest possible cactus farm

Not a build, a ceiling. ilmango takes the standard cactus farm and upscales it until the game itself is the limit: a circle about 256 blocks across and 384 tall, running from the bottom of the world to the top, opening a series on the maximum possible rate for every farm type. The design is the ordinary fence break cactus farm repeated until the server tick is the bottleneck, and most of the video is about buying that tick back rather than about cacti. It costs 1,144,832 cactus to build, so you need a cactus farm before you can build the cactus farm.

mikey88888 and PanNic97 both suggest end portal blocks in place of the nether brick fences, so items vanish into the portal the instant they break off. MethodZzS says it does raise the rate but makes the build far more tedious.
loniout4666 found cacti landing on top of the fences in rare cases, and asks whether the half slab version leaves them stuck between slab and fence.
THEMithrandir09 would automate the construction itself, dropping the sand with flying machines and casting the outer walls as cobblestone from lava and water.
The_Lazy_Squid corrects a common assumption: random ticks are decided server side, so standing more players near the farm does not speed it up.
Lithium is doing the heavy lifting. ilmango runs it and sfisher923 notes it took the overworld from 110 MSPT down to 30, so vanilla performance is not what the video measures.
htgnef puts the slab count in perspective: 4,319,648 smooth stone slabs is 2,500 shulker boxes, needing 47 double chests of shulkers just to hold them.