from my understanding, Season 5 is where they run out of book material/dialogue originally, which is why Stannis plot line falls apart drastically and Bran is not even in the season at all, and outside of creating the High Sparrow plotline, was just used to reach the plot lines you saw in Season 6 with resolution
so basically they dragged their feet through season 5 because they knew how some of the plot lines would end in season 6 so they just needed to some how get to season 6
for me personally
season 1-4 was great
Season 5 was DECENT-GOOD
Season 6 was good-great
Season 7 I know most people hate this season but I thought it was DECENT, but I can see why people hate it, basically started the whole teleportation/time travel bull shyt that didnt fit in the series, but I was still entertained during some moments of the season
season 8 i try to pretend didnt exist, I legit wanted to throw my remote at my wall or tv during multiple moments of this season because it was that bad. Just horrible TV through and through, not 1 redeemable thing outside of the soundtrack. I remember saying, aloud, even by myself "What
this shyt fukking trash" while actually watching it live