How to not have slow processing speed?

I play 15/10 rapid game. Opponent has 18 minutes on the clock, and I have like 12 seconds. lol. And yet I'm losing. 

I followed the biggest tip of all, and I did a lot of survival rush puzzles and have 1850 elo in that. but I still remain 800 in blitz.

I don't have an answer but I'm going to follow this thread to see what higher rated players have to say. I'm in a similar boat as u - delta between my rapid(1200) and blitz (450) is huge. and my puzzle rating is quite good as well (though that one is largely irrelevant).

My tip as a 1400 rapid player is that blitz can be dangerous to chess improvement. Daniel Naroditsky used to recommend blitz for building your instincts, but it's easy to tilt, to lose confidence, and for that to negatively impact your rapid game. I typically play it one night a week on stream, but recently decided even for that, it's not worth it for me.

Puzzle rating is not equivalent to rapid/blitz/bullet rating. And at any rate, those puzzles won't automatically make you a better chess player. It's just one, pardon the pun, piece of the puzzle. I'd say if you are doing puzzles, sorry chess.com, use lichess because you can filter them by opening, and only play puzzles that you would actually see in your games.

15/10 is what I prefer playing - I feel like it's a good balance. If you feel like you are burning way too much time, limit your search for moves to 2 or 3 candidates and choose the best one. Stick to openings and pawn structures you know well, and try to keep things simple - often times simple chess gets the job done. No need to do any crazy piece sacrifices, especially 1200 and below. It's not needed. Just make sure to castle before your king gets into trouble and once castled, don't push pawns in front of your king. Like I said, don't do anything too complicated if you are having time trouble.

I'd recommend sticking to 1 opening as white and 2 as black (a response to 1.d4 and 1.e4) and learning the theory, the main lines of those openings. That way you'll have a good idea what to do in the first 10-15 moves, what the main ideas are, and how to respond to your opponent. I looked at some of your games, and in those games you'd spend more than 1/3 of your time in the opening and that's too much.

Play bullet

Puzzle rush survival is not accurate; I could go to 3000 Elo on puzzles before I lost.

You should play blitz NOT bullet to improve processing speed

From the progression of your rapid rating, you are already doing a good job improving, just stay with it, it'll improve naturally. Do keep a healthy balance between Blitz and longer time controls. The best way to develop tunnel vision and shallow thinking is to play too much Blitz & Bullet.

I find that cognitive speed and pattern recognition availability also have a lot to do with a healthy lifestyle outside chess, that often gets underestimated.

Some people specialize in speed chess, but most of the strongest speed chess players are also the strongest chess players at longer time controls. It’s probably not that you are “slow processing”, but more accurate to say that others are “faster at processing” (whether they are playing good moves or not is a different discussion). Strong speed chess players move quickly mostly from already knowing what to do without needing to think much.

This could be high levels of pattern recognition from lots of chess playing experience, or knowing lots of opening theory to save time there, or similarly knowing theoretical endgames well, so they can convert efficiently.