Shed some light on JavaScript loaders?

Why do browsers take so long to load modern templates with loaders? I have used HTML Templates from sites like Template Monster, and I deal with this from time to time. A lot of times, it is different on brands of browsers (e.g. Chrome, Firefox, Edge). One takes a long, and the other does not (or vice versa). However, this could be my inexperience with these JavaScript loaders. I want to get to the bottom of it because I know this is not the intention or purpose of why it has a loader. I am talking about how sometimes the page hangs, and the loader spins for a long time. I even experience situations where it does not ever open a page. Then, I move to a different browser, and the page pops right up. Could someone shed some light on this for me? Thanks