[Dilligaf]

As I understand it, 1080i is interlaced so that it alternates between sending even and odd rows in each frame. Therefore, only 540 lines are sent each frame but it is so fast that the illusion of 1080 lines is there. Whereas 720p sends 720 rows in every frame (better for sports with fast motion). This would make 720p more heavy to render than 1080i.

I also confirmed that all my channels use dolby digital (even 720p) so it's not decoding DTS audio. My next step would be to try a different back-end, although MediaPortal works fine to my main HTPC.

But you're not taking the width into consideration, 1920 vs 1280, 1920 is 50% more than 1280