I did a test to check out my addons cpu usage, sadly bigdebuffs seemed to be the worst one of my addons.
This is on classic wotlk, bigdebuffs v11.2, default settings.
Tested during Felmyst in SWP, the only debuff that was shown was a Viper Sting attached to my shadowed unit frames target&focus frame. No debuffs were shown on my nameplates, plater. (This is because nothing was cast that would show up, jfyi).

Had this in SWP last night too. Had to disable the addon, that was out of combat not even targeting a mob.

can confirm i have this same problem ever since wotlk pre patch. particularly in 25men raids.