Hoppers. they're super useful for all kinds of things. unfortunately, lots of them can cause lots of server lag. the solution? put containers above any hoppers that don't need to pick up items. what this does is stops hoppers checking to see if there are item entities to pick up above the hopper.
https://www.youtube.com/watch?v=qEi-tKrxDNk This video shows exactly how this effect works.
Notably: 27,000 hoppers were placed into a multiplayer server. the hoppers used 93% of the server's CPU power. then, furnaces were placed over every single one of those 27,000 hoppers. the CPU usage reduced to 15%. This is a MASSIVE improvement in performance, and if lots of people did this with their hopper-powered devices, it could make a HUGE difference in our how fast the server runs.