Any type of slow chess will help you improve better than rapid. I can't speak on correspondence specifically I don't play it but I think 90/30 or maybe 45/45 games would help your overall game better. Real games are finished all in one day not two months lol.
Does Correspondence Chess improves your game more than rapid chess?
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Of course correspondence chess improves your game faster than rapid chess. When you are improving, you need time to think and process new ideas. Long games are also important. Blitz is pointless unless you are a GM trying out new opening ideas, or unless you are drinking whiskey, then it's quite entertaining.
Ey so I have noticed that Correspondence Chess is improving me at a decent level of speed that is satisfiable, unlike my 15 minutes games on ICC. For further informations, my rating is around 1950 ICC, and that might be why it's harder for me to improve; btw I had only joined Chess.com for 3-4 days, so my rating is only 16xx (surely it will improves, since I have yet lost any of my matches).