How to reduce lag

Do not use huge amounts of item frames.

• Decorative frames here and there in builds are fine, unless you decide to make ceilings out of them
• In sorting systems please use the actual blocks as much as possible instead of frames

Do not use more hoppers than necessary

• You can disable hoppers from "checking" for items they can suck in with a composter
• If you use really a lot of hoppers it would be nice if you could turn the entire hopper chain off

Try to avoid constantly running redstone clocks

• Use clocks that can be turned off, or switch on and off automatically wherever possible

Do not breed excessive amounts of animals

• Keep only a few for breeding
• If you need to breed a lot, you can use Twisting vines from nether. This bypasses collisions and entity cramming

Make sure your storage system sagely destroys overflow items

• You can shove those to overflow chests
• Other than that, cactus, lava and fire are your friends

If you want to build something laggy, build it in a remote location where it won't be constantly loaded by others

• Industrial districts are great projects