I tried to start a new game, I had that random problem of low fps even when the game did not use all the resources and indoors it almost did not happen, I tried to deactivate the odd mod to check it may be one that modifies lighting or the terrain and maybe an "ENB", you can also look for some utilities to repair game errors in the bug fixing section. "google traslator" https://www.nexusmods.com/skyrimspecialedition/mods/categories/95/