I left most of the side quests unfinished in soul cairn and I was wondering if there was another, faster way of getting back to the portal, or will I just have to trek back through the undercroft?

In the laboratory where the portal is, there is a normal door to a balcony. This balcony is a discoverable location that can then be fast traveled to.

wish I knew that before I left it , thanks though, I will make sure to discover it this time round

Its a bit odd for a discoverable location, i think its there specifically.