It's called the Glicko Rating Deviation (RD). When you have very few games, your RD is high and you will see larger rating swings. As you play more games and your rating becomes more stable, the RD drops and the swings become smaller. Long gaps between games can also make your RD increase.
https://support.chess.com/customer/en/portal/articles/1444777-how-do-ratings-work-
At first your rating is provisional because your strength is unknown. For that reason, your first wins give you more points and losses take away more. Over time as your rating stabilizes into a certain area, you gain/lose less points per game because that's expected to be your real rating.
So after reading the article with the link attached, it sounds like as you play more games, your elo slows further and further because it gets more accurate... So how are there people with elo's around 2000?
They either won a lot early on, so their RD stayed pretty high or they continued to win against higher rated opponents, frequently enough to increase their rating.
Play unrated until your RD increases again.
So I started playing on here about a month ago, and when I started out I'd get like +20-30, sometimes even +50 for a win, but now even if my opponent is 100 points higher than me I still only gain 10-15 points... what happened? Why is it doing this?
I usually play 30 min Rapid, and 10 or 5 min Blitz, nothing else.
Probably because you are playing rapid and blitz. There is a recent article by GM Gregory Serper. He recommends you take about 15 - 20 minutes just to play the opening. Can't do that in fast chess. Fast chess is full of errors and inaccuracies. I don't understand why anyone would play fast chess.
So I started playing on here about a month ago, and when I started out I'd get like +20-30, sometimes even +50 for a win, but now even if my opponent is 100 points higher than me I still only gain 10-15 points... what happened? Why is it doing this?
I usually play 30 min Rapid, and 10 or 5 min Blitz, nothing else.