I've had this problem for some time now, and could not figure out what the hell was going on.
I would get massive frame stuttering and extreme FPS drop in small interior locations or when looking in certain directions. This problem has plagued me with ENB presets over the months.
Until I was in a cave and staring directly at a camp fire. Only when looking directly at it would my FPS drop 20-30 frames and produce stuttering. So I sat there staring at the fire and went through every ENB option to see if I could find the culprit - Soft Particles
Turning off Soft Particles completely removes the problem for me, it also changes the textures of the fire embers. I'm not sure if it's related to a texture mod I have installed or not. I have removed "Realistic Smoke & Embers" as well as "Ultimate HD Fire". The problem is definitely "Soft Particles" for me.
Soft Particles: OFF - FPS 60 - Fire displays glowing embers
--------------------------------------------------------------------------------------------------------------------------------------------------------------
Soft Particles: ON - FPS 40 - Fire does not have glowing embers
Strange bug, or I have bad settings?