Epic is aware of the problem, it is tad bit complex to fix.
u/FreightTrainUSA, Me and a friend of mine did identify some of the causes:
Getting close to a radar can cause the performance issues to happen
Getting close to the frisbee launcher can cause a lot of performance issues.
Getting close to a survivor can cause even worse performance issues. Especially noticeable in Rescue The Survivors mission.
Getting close to an orb, a storm chest or any other encounter type can cause the FPS drops.
Elemental husks spawning from patrols and during defense in any defense-type mission can cause FPS drops.
TL;DR: Basically it's general UI causing such problems.