Finally got the toons to almost 90 through pure dungeoning but holy crap 85-90 takes a loooong time. It's on random dungeons and I haven't been watching them at all really. But after leaving them for around 7-8 hours and only gaining like 1-1.5 lvls at 88-89 I decided I'd watch them to see what's wrong, or whether it was just slow xp due to no rested/heirlooms.
Turns out Gate of the Setting Sun is just bad. The 3rd boss I caught almost every one of them standing in the green shit, the melee in the group constantly dieing to the frontal cleave, the healer being ravaged by the little adds for most of the fight which caused a lot of deaths. The last boss was the worst though, for some reason the melee jumped up but half the time just did nothing, and the ranged jumped up and shot the adds rather than the weak spot. Since the weak spot didn't get DPSed, the phase never changed and the room got pretty much filled up with tornados. The healer pretty much stopped healing and was running around dodging the tornados, but it's movement was just horrible. The casters did the same and was spam interrupting casts due to it. I ended up just turning it off and put them on Kicks for the last level.
Also caught them in Siege of Nieuzau on the first boss the whole team standing in the yellow shit in the middle of the room and dieing to having huge stacks of debuff.
If I'd do another team I would definately put them on specific dungeons that they can get through well once they are past 85.