Biggest causes of lag

In my datapack, I consistently see at least 10 ms (millisecond) per tick, with that value sometimes going up to 30 or above when certain functions are continuously run. My understanding is that going above 50 ms per tick will begin to noticeably lag your world, and I haven't even tested it in multiplayer or with other datapacks yet. What sorts of commands might be causing these issues, and how can I generally make my pack more efficient?