I saw the same on first running 1.6 with an old save - real degradation in performance, especially during driving. At times like treacle, then suddenly good, then suddenly bad.
Running perfectly now though - solid 60 fps at 1440p on Ray Trace Medium preset - really much better than I was getting on 1.5.
What seems to have fixed was, after reading some other comments on this, resetting the settings in Graphics and Video settings menu to default, then quitting and restarting (not sure if that was necessary but hey), then re-adjusting the graphics settings from scratch (for me that was basically putting vsync back on and choosing Ray-Tracing Medium preset).
That's all I did and since then (touch wood), it's been rock-steady, no glitches at all with map or any of the old problems.