I expect the additional lag to be incurred mostly because of additional mechanics. HM Bestia is already pretty hard on my frame rates, and I'm running it on a potato machine in the most potato settings that are practical to a healer: lowest graphics, no nameplates (I found that it's easier to do without nameplates as a healer than as a tank/DPS; when I tank/DPS I'll turn on nameplates on hostiles only, nameplates make my life so much easier when mechanics call for a specific kill order, or to hold aggro on a specific set of hostiles), no flytext.

For the purposes of this question, assume content is run in 8-man. For those among you that ran HM/NiM ops, are there fights that lag more in higher difficulties than in SM? If so, what additional mechanics are in play?