Randomly will get 5-10 second long lag spikes during the Mug’Zee fight with a raid size of 20-25 people, my FPS and ping are the same as they always were and never had this issue before during other circumstances aside from this fight specifically.
If it’s only that fight, and you use addons, check your boss mod (bigwigs or dbm) for an update. Alternatively, could be a WeakAura?