@TheOnoZone Hi, Ono! 46 years old and learnt the game in my 40s. My experience has been quite similar to yours, in that my rating has been higher in slow time controls compared to fast.
But one strange observation I have made is that performance may also be related to opening choices. Some time ago I was in the high 1600s in lichess classical and in the 1300s lichess blitz. At the time I played the Reti as White and Pirc/KID as Black. I decided to switch everything to 1.e4 as White and e4 e5 or d4 d5 as Black. I did this mostly to broaden my horizon, as I felt that I was missing out on so many famous openings and structures and positions by playing offbeat openings. I also decided not to study opening theory except opening principles and analyzing and learning from my games.
The result after a couple of months of this is that my blitz rating is up 100-150 points and my classical rating is down 100. This is a bit confusing to me, but my theory is as follows:
- 1.e4 leads to more open positions where anything can happen. I can beat much stronger players due to tactics (which I study a lot of), but of course also blunder early. 1.Nf3 often gave me an advantage out of the opening, but the positions were less intuitive to me, and I needed more time to calculate the best move in the middle game. Hence I was bad at blitz, due to being outplayed in the middle game or getting in time trouble, but better at classical, as I had more time to figure out how to convert.
- For Black I think the problem with the Pirc/KID in blitz was that I lost a lot of games quickly due to kingside attacks that I was able to counter better in slow time controls. With d5/e5 I don't lose quickly quite as often.