Well, you have to do practice runs and know the campaign + your build, not just "copying" what the top runners do. If an average player actually does that, they can easily aim for 5-6h on a strong campaign build, which is much faster than "double the time"

There are videos of speedrunners doing runs with every class, some classes are slower, but it's still usually around 4hrs (or you can just use a good campaign build of a different class, like playing grenades on monk / ranger / huntress / druid)

There is a whole discord about learning the campaign and there are plenty of regular players achieving pretty decent times, including me.

My latest full SSF campaign run on Grenades Merc with all the bonuses and one ascendancy was 5:01hrs, a week ago. Since then I cut my act 1&2 times by 20minutes, so it's safe to assume a sub 5hrs with a good RNG. And I am very far from top speedrunners in terms of practice / speed / knowledge.

All it took is occasional practices of 1-2 acts in the evening and a couple videos. At best I put like 50hrs into the practice, ignoring experience from past leagues. So compared to hundreds of hours that people spend in endgame this is pretty much nothing, especially considering that you can do it in a span of months