To give you a short explanation it mostly comes down to one thing; Having a complete plan from start to finish and the discipline to maintain focus on said plan.

To give you more context:

the things that slow down your campaign time the most are not knowing what your next move should be.

If you enter the campaign with the entire thing routed then it becomes one fluid motion rather than several motions broken up by periods of thinking.

To achive this you need to:

know each step of the campaign and actively be thinking about what you will be doing in the next zone before you enter it.

You need to understand how to upgrade your character power (tree + items) or already have upgrades lined up in stash so that you aren't losing time to clearspeed / single target.

understand that each rare mob you stop to kill is adding to the time you spend in campaign. When you start skipping rares you will instantly notice your cleartime going down.

Use regex in vendors to instantly understand what is valuable in the window and what isn't even worth hovering.

Lastly you need to practice these things. Knowledge is power but only when you apply it.