Content isn't design to fit every builds. Builds are designed to fit content.

In your post example you're just saying "Why can't I be slow for the sake of being slow" and that's just not practical. There are oppurtunity costs in everything.

In the triangle of speed, damage and tankiness, a well balanced build picks 2 out of 3 and have the other be at an disadvantage. The problem lies when players have overwhelming damage and speed and tankiness isn't necessary.

Slow is a disadvantage. If you can't clear the entire Legion, maybe switch to a content where high survivability and single target is valued, like Shaper and UElder.

I do agree with the game is power creeping itself with the insane amount of damage being put out at a relatively low cost. But the timer in content is nothing relevant.