[Baertram]

afaik there is no way to detect that by any given lua or ESOui lua API or function.

And your FPS decrease most likely happens the longer you play. Try a /reloadui here and there and it should free up ressources, like created controls and other lua objects, internal tables, map pins, markers in inventories etc.
They should be re-created then once you really need/use them.

I'd also disable just addons you do not need like raid addons at overland, and vice versa. Carfting related stuff if you quest.
Price related addons (or libs like e.g. LibHistoire) and guild addons if you are doing dungeons, overland quests.
And so on.

You can use Addon Selector to create profiles to easily change the addon packs depending on your needs. The more addons enabled, the more lua operations are done, the more controls get created, the more lua tables stored and updated, the more memory consumption, the bigger the SavedVariables get, the longer the loading times, ...