When I came back to the game a month before Shadowlands release, I got into one of these speed run leveling dungeons and as a restoration shaman trying to learn the game again after not playing it in 9 years, my experience mirrors the OP and was barely able to keep up. It was an experience that made me stop being a healer and switch to enhancement shaman.
I used to be a hardcore raider, and learned to solo a lot of group content as a restoration shaman but after that experience I stayed a solo player that doesn’t raid, and I stay out of Mythic content so not to ruin other people’s experience in that content.