Think I may have found the reason for the huge spikes in FPS...and you're not gonna like it

Above and on ground in cities turn terrainlod slider back to 200 and set trees and bushes to medium, leave objectslod at 200
Above ond on ground in naturescenery set objectslod slider to 100 and leave terrainlod slider at 350 or 400 and set building to medium

Maybe this will aid on the queeste to stuttter free experience