I was guessing on headley's comment: the fact the problem appeared also with the trial expired, lead to believe it's not something related to buildings because, after the trial time, there are no buildings anymore.
However, it's quite a difficult compromise to choose between being constantly at an high frame rate, risking stuttering when different level of detail kick in, or having a constantly lower frame, with less stuttering.
Considering all the sort of addons people might use, it's basically impossible to guess performances on the actual user's configurations. While we are quite sure the frame rate is very good, moments of stuttering are way more difficult to anticipate, because they also depend on everything else you have in that area in a specific moment: default scenery + addon scenery (mesh, vtp terrain addons), addon ai traffic, etc. Tuning for less stuttering on one configuration, might simply create a bigger problem on another one.