21 mods including libraries, plus a few resource packs. Every time I enter a world, it lags, but before it was okay. I don't know what to do, as I removed some mods but it isn't making it less laggy. What can I do?

Try looking at the "My game is lagging, how can I find the culprit?" section of the FAQ:

https://forums.minecraftforge.net/topic/125488-rules-and-frequently-asked-questions-faq/#:~:text=My game is lagging%2C how can I find the culprit%3F