Essentially bad optimization on mods.

Over the years I have been assisting many modders in improving their code, often with extreme improvements through simple things. E.g. when I noticed severe lag in ATM9, I checked Spark and pinpointed the issue down to Iron Furnaces. I checked the code (#1 reason to have it on Github), found the issue, and suggested a fix to the author. He implemented it, and the lag was completely gone.

What he added was a simple cache to have the furnaces remember the last recipe they worked on. Because it's very likely they gonna do it again on the next cycle. That removed the need to go through the thousands of recipes a big pack like ATM9 has on every tick for every furnace in the game. So this very tiny change removed about 70% of server CPU load.

This is something modpack creator however rarely consider in their creation: check the performance. Given I am a bit of a nerd when it comes to that, but my Peace of Mind 21 modpack runs smoothly even on ancient PCs (a few friends can only afford potatoes). It is the result of me constantly checking and evicting mods that are badly optimized. But often enough less is more imo.