Here's what happens when u play fast and think ur improving. As with anything in life, any career/skill, etc. if u want to be proficient at it, u first need to learn to do it right BEFORE u try to do it fast. If ur doing it wrong but fast ur only reinforcing bad thoughts, bad habits, bad technique. I'm in construction and I've seen many people over the years attempt to impress people with their speed. However, if it's wrong and it looks like fuggin chit when ur done what good is it? It's the same with chess. If ur playing fast making mistakes over and over then ur just reinforcing poor playing habits. U still suck at chess but u just suck faster at it. Not impressive. I hope I've explained this so that it's easy to understand. Good luck with ur speed chess.
Edit: about good players playing blitz/bullet. They might play a lot of speed chess now BUT..... they didn't get good from playing speed chess. They got good FIRST from studying, training, playing slow tournament games, analyzing their slow tournament games, thinking deeply about positions, then they automatically became good at speed chess based on their thorough foundation of knowledge AQUIRED FROM SLOW GAMES.

Try it even if it's only for a couple of days.
do you really expect us to take advice from someone rated below 700 rapid