this is unplayable, i had a teammate who was gadgets using stealth and the instance became unbearable to play in? i dont know if mental is the same way but for sure gadgets stealth is a problem in survival mode.
Apparently it gets like that if you're in the instance too long and it's existed since back in AM era. Just have to relog to fix it or not use stealth if you can't afford to relog.
Stealth and invisibility have always lagged instanced and especially SM, maybe its something that has to do with the size of the instance since Trigon SM was always really laggy aswell. Closing instances and opening again always helps. Another thing that used to create lag/fps drop was switching items inside inventory. If someone in group did this it would make you lag but I cant remember if this was fixed or if its still in game. Just something to take into consideration.