Minecraft wiki entry for Tick, associated text: Hoppers constantly try to search for items above them. Cover with any item with an inventory slot or a composter, as this stops the checks for items. Alternatively, use water flow-based transport which is faster in bulk.
Redstone machinery. Redstone components, especially redstone dust cause excessive amounts of block updates and lag. Disabling redstone contraptions and clocks when not in use can help mitigate this.
Mob AI. Use torches to guide hostile mob spawning. Use more efficient farms for animals.