Having multiple issues with unity. suggestions?

I have been learning unity for some time and for the most part i have the basics down, such as creating a terrain, scripting, adding textures, and so forth. Recently, when i got on, so many things went wrong. First of all, my FPC capsule kept falling through the ground, which was out of no where. I ended up having to add a world particle collider, which was never suggested in any replies among the hours i wasted searching everywhere. Now, whenever i want to do a test run, like walk around my map, i experience MASSIVE lag. This has never happened either. It is as if Unity is just mad at me, and wants to piss me off. Well, it's working.

And please don't suggest "putting the capsule above the ground, blah blah blah" i know all of this. I'm not a FOW when it comes to unity, as i stated before. This is so frustrating and I do not know what to do. Now, before i lose my mind and delete unity all together and start over, will someone please explain as detailed as humanly possible why I am lagging hardcore, and why my capsule just feels like falling through the ground whenever? thank you so much in advance.