The game should for fun and not a stress inducers. Play it to the level you feel comfortable. Don't let the elitist speed runners ruin your game experience. If you are tank or healer chances are you will run into them one of these days and they will try to force you do things you are not comfortable with. The proper response is just hold your ground keep doing what you are comfortable with and they will have to yield, because they can not push string no matter how hard they try or how mad they get.
If you are a DPS. most of the time they will just ignore you, but sometimes they will decide to be some busybody ninny and they will probably berate you for not optimizing your rotations or what not (BTW they think yelling at people with their condescending attitudes is polite advice because they've take their precious time to explain stuff to someone else.) They will also make up euphemism like "improvment" or wanting (i.e. forcing) you to improve. Don't let them stress you out, or guilt you, or force you to do stuff you are not ready for or simply do not want to.
Don't feel bad when the jerks like that start acting up. It is unfortunate that some dungeons are 4 players and it is not that uncommon for the 3 of them to know each other. If all 3 are speedrunners they are abusing the duty finder and just want to take advantage of the tank or healer that just happens to be available on the duty finder. The 3 on 1 situations like that are really unfair, because the majority of the player base is not like that and does NOT behave that way. If one them keeps harassing you report that guy to the GM. If they abuse the vote kick option, report all three of them. Also don't let them force you to skip cutscenes you are seeing for the first time. Additionally if you don't want to see spoilers and don't want to have to study videos of dungeons before your first encounter you should not have to. You have the right to see the game as fresh and new, but these elitist will even disparage you for that. In short, do NOT give into these bullies.
And this kind of statement is just all kinds of irresponsible and totally unclear. How the heck can the new player, casual players, or player so casual that they are basically non-players be even what is wrong. They can't be begin to tell top from bottom and yet they are guilty of some unspecificied accusations. This is just utter BS. Not all equivalences are valid. Just because people can debate whether the earth is round or flat, doesn't mean the "earth is round" guy is wrong the same way like "the earth is flat" guy is wrong. SE decisions to make the game more accessible more inclusive as draw in casual players is correct one. Trying to impose exclusivity like the way elitist do is flat out wrong.