If the instance was intended to be killed 1 mob at a time, there would be locked doors everywhere that won't open until you kill the current wave. Most dungeons only have about 4 or 5 of that lock on average and many of the trash mobs are obviously skipable. You can't say that the game intended you to full clear every dungeon when there is all of this option to speed run and save time.

As for the people that go too fast for your pace, just tell them to slow down or you won't be able to keep up and the party will wipe. Nothing's wrong with being assertive for your own sake. You have to understand though that most people are running the same dungeon for 50th time because the game basically force them to do it in order to progress their relic quest so it's pretty common for people to go in instances, say nothing, turn on netflix and faceroll through the last boss and treat everyone else as an NPC.