Tal,Fischer,Kasparov,Karpov,Petrosian,Bronstein and many others too,were very strong blitz players - if blitz is so harmful , why they were in a serious chess top players too ?
Why I hate Blitz or Lightning (Quick) Chess

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It's not harmful to be good at it lol. If you're a badass in classical time controls, you're not going to be a pushover in blitz unless you have some sort of mental block to overcome.
It's harmful to play blitz 5 hours a day. I wonder which in that list played blitz regularly? And IIRC Kasparov never went to the Amber rapid and blindfold tournaments.



Not too long ago, I beat the founder of our local chess club 5-0. Then he challenged me to a blitz game. Although I enjoy rapid chess, down to 10 minutes, I knew that it would be foolish for me to play blitz with him. Bad chess player that he was, I knew that he would probably win.
My experience in life is that if you beat someone in a boxing match and then they invite you to a wrestling match...watch out...red flag.
Well, the ratings haven't stayed the same. I think most people lost 1 to 2 hundred rating points in the faster time controls between 2010 and 2012, so if you stayed the same then you certainly improved.
But improvement isn't completely arbitrary, i.e. you usually feel like you've improved in some way. Maybe look back on some of those old games and see what you think?
Also, for most people, if all you do is play for fun, then progress will be very very slow (if at all). You have to analyse and study to get better!