What is happening while I'm sitting at the load screen? [closed]

When loading into a game sometimes it takes a real long time due to just one player. Often you hear the "toaster" jokes once everyone gets in game referring to the assumed poor quality hardware of the slow loader. I always assumed the long loading times had more to do with network latency than their actual hardware. But that's just because I have no idea what's happening in that time frame. So is the wait time actually because someone had a slow computer, or is it because they had slow internet?