higher grant more xp but they take 20-25% longer to do in general, and that is if you do not wipe to them.

The first 2 are just brain-dead easy and can be zerged through literally ignoring most fight mechanics. 2nd boss of Crystal Towers for example... how many people see the jumping platforms part of that fight... or even know they exist. Another one is how many groups have you seen operate the lightning tower things in the KB fight in Lab. Now compare that to say weaping city or the more recent raids... if people ignore mechanics in those its probably a raid wipe or at least a full party wipe.

It is all about XP/HR. The increased boss XP does not come close to matching the lost time. The harder ones should Scale up to be more xp. Aka if it is takes 25-30 mins on average for a higher lvl raid and lab takes 15-20 you should get ~40-50% more overall xp to warrant the increased time you spent and difficulty for the raid. In my opinion that is the only real fix to the problem.