I keep hearing blitz chess is bad for you but the I've only been offered two explanations for why that is. The first reason I've been told is that it ruins your ability to focus since you it forces you to play fast so I suppose they say it makes you less patient as well. The second was that blitz is not a very efficient way of improving chess since you can't put as much thought into it in general. But surely that isn't the only reason people say not to play blitz right? After playing blitz I still maintain my ability to focus and as long as it doesn't make me play chess worse I should be fine with reason 2. I really want to understand what this whole deal is about since with my personality if I play blitz or play around with chess in an absent-minded or quick manner I get paranoid and start thinking "Oh no, am I making myself worse at chess or something like that?".
I am answering only as an expert player, so who am I to answer this, but I think that "blitz ruins your chess" was a very popular view some time ago rather than nowadays... Anyway, given the very limited time at your, and your adversary, disposal, in blitz chess a player is not generally searching for the best move(s) but the one that gives you most chances. This may encourage, in the long run, a player to "stay away" from very deep considerations, tactical or strategical, and this is ultimately not good for improving chess strength. When you face strong players with plenty of time at their disposal you have to find the best moves, because they haven't 30 seconds on the clock or so, unless they are in zeitnot, to answer you.
I am answering this as an intermediate level player. Making good moves in fast chess results from remembering positions or patterns. The repretorie of strong moves in a given position developes when you have already evaluated similar positions and made similar moves. You will make strong moves only when you have thought through more number of outcomes, which is a slow process. Therefore, real learning comes through slow chess. You will be good at fast chess when you are good at slow chess! Only playing blitz (10 minutes or less per player) is a waste of time.
Playing blitz chess got me in a rut. I played simply to win the game, or to confuse my opponent, or to get in wood-shifting positions where I wasn't thinking of strategy. I was just playing for the immediate move and skating on what I knew instinctively.
That said, playing blitz chess is very worthwhile if you have a plan and use it to try out new ideas or openings to get quick ideas of where you go wrong, or what variations give you trouble in practical play. If I got back to more regular playing, I'd take notes on my losses.
But if you start playing by rote or just for short-term competition or excitement, it may damage the long-term view. For me it was too tempting to go in for gotcha moves instead of more careful planning.
I think that all this talk about slow chess allowing one to find "the best move" is nonsense. Read the book Thinking, Fast and Slow. I suspect that blitz chess trains one's intuitive chess mind while "slow" chess trains the analytical chess mind. They're both valuable. Magnus Carlson says that he always "knows" immediately what his next move should be. When asked why it then sometimes takes him many minutes to make that move, he smiled and said that he had to analyze the board to prove to himself that this move was indeed the right one. "Almost always", he said, "it is."
It's to do with the reward system.
If you play 10 blitz games (e.g. on playchess), you'll be matched against similar players of similar strength and should expect a payoff of 50% (equal numbers of wins and losses with some draws).
By the nature of blitz, you have limited time to digest and analyse the position. Therefore you could attribute a win to luck, a draw to time and defeat to bad play. In reality, time is the biggest factor.
The best thing about playing a long play game where you have time to analyse is that in general you have time to digest and learn from where you went wrong or could have improved (even if you win). Therefore it's a learning experience which you develop from. After a blitz game you'll generally just go to the next one, digesting nothing and the whole thing begins again!
Chess is a game of insight. Short time controls don't encourage insight. Therefore, blitz and bullet aren't chess. Or so goes one popular line. ;)
I'd say blitz has its pluses, but only after you have mastered the long game. Magnus is good at blitz because he is good at "real chess," and not the other way around.
I'd also add that engaging in blitz marathons shares the dynamic of addictive gambling. You're in it for the rush rather than the quality of play. Careful not to get sucked into it.
Playing blitz chess gives you a lopsided set of skills. It does improve your ability to think quickly, and to find a "good enough" move for most situations. In this regard, it is good training for the times (in tournaments), when you get into time trouble.
The problem with "blitz" is that real mastery does not come from finding "good enough" moves, but rather finding the best move, time and time again, which requires slow play. Here, blitz is counterproductive, because your highest achievable level is based on the best moves you can find. Using "good enough" moves keeps you below that level.
My experience has been that playing blitz creates bad habits that carry over to my slower games, such as rushing my moves and missing obvious attacks by both players. And of course unless you're a real chess savant, it's impossible to analyze all the pertinent combinations within a short period, leading to superficial play. Of course, your opponent is under similar constraints, so mistakes on both sides will abound. If you want to play better at slower time controls, you must develop the ability to think deeply into the position, which blitz will never foster. To play blitz well, you must have a firm grounding in the basics of chess first. That can only be accomplished with study and serious play, which implies slower time controls and subsequent detailed game analysis as well as study of master games. I'd start out with a chess manual to be sure I understood the basics and then progress from slow to faster play as you improve.
It depends on the exact time control and type. I went straight from zip to 1750 USCF by playing weekly at the club in their lightning tournaments. Every 10 seconds a buzzer or bell rings and you have to move immediately.
Games looked more like movies. You had enough time to plan decently and to make your move. They taught a lot more than playing one slow game in that evening. Plus the fun and incentive of playing a full RR and seeing your results.
I blame the electric clocks and all the oddball add ons and delays for making blitz chess bad for most players trying to improve.
Perhaps a slower time control would have been better e.g. 20-30 seconds per move. But ten was the standard back then before electric clocks. The alternative was SD10 using push button mechanical clocks.
I would say that now playing G15SD would be near the sweet spot.
My friends now want to play G10 +delay4. They often run out of time near the end while I usually have a few minutes left.
Playing exactly ten seconds a move trains you to both think and to move promptly. I suspect that regular OTB trains people to take lots of time and then they get into time trouble and have problems. Too many people do not pace themselves and use their time wisely.
Clock management is a key to playing OTB well. It does no good to get an advantage by taking lots of time early only to lose the game later on when you have time trouble.
While blitz may help with play during time trouble, it also tends to make you impulsive and move too fast early on.
My experience says if you want to do well at OTB then do not play blitz but do play at faster time controls like exactly ten seconds per move or at G15SD. G30SD would be the slowest I would suggest if you truly want to improve. Perhaps if your clock does this you could try delay5+5second add on per move to try to simulate the exactly ten second time control. The problem with both add ons and delays alone is that when the other person moves immediately you do not get the extra time to think like the old must move only on and not before the ten second bell chimed, and that it is possible to save up too much time to be good for training purposes.
There are different styles of playing blitz, and it is true that in most blitz games cheap tricks come into it sooner or later. But I have known players who approach it differently. Say as White they play an Exchange Lopez, trade all of the pieces and promote a King-side Pawn. In all situations, they try to play some favorite line with a strong strategic plan where they know exactly what they should be doing. This can be very hard to play against.
This would not suit everyones style, but it is a form of blitz that reinforces your approach to the regular game.
This answer uses some ideas from Andrew Sainsbury's answer. I'll make a guess. According to https://www.wikihow.com/Speed-Run-a-Video-Game, sometimes when people start speedrunning a game, the speedrunning can transfer into other tasks. There was a guy with username EnNopp112 on www.twitch.tv who said his speedrunning transferred to other ares such as making a sandwich.
I think that similarly, the habits of playing like you're playing Blitz can transfer to matches with a longer time control like a 90 minute base time with 30 second increments.
I think that in Blitz tournaments, you have less time to think. You can expect that your opponent also has less time to think as well. In general, when you have less time to think, you cannot think as far ahead. I think that sometimes when somebody is playing Blitz a lot but no chess with a longer time control or no clock, after they immediately start playing a chess game with a longer time control, they still have the habits from when they were playing Blitz. They don't take advantage of the time they have to think further ahead whereas their opponent does. If they keep their habits from when they were playing Blitz in a regular tournament, that might cause their rating to go down.