Yea rapid allows more study and thinking than blitz, but I am like up 3 or 4 minutes almost every game cuz the people slow as a tortoise (I played lots of blitz so)
800 ranked player looking for advice
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For improvement it's just about playing longer time controls and studying every part of the game. I think tactics and solid openings are important to get to 1500+. The game is just easier if you have solid openings(ideas/knowledge/understanding - not just memorisation).
If you want to improve you have to go into a game with the intention of learning something and following up on that after the game. Save important ideas and go over things you don't understand until you eventually do.