gajit - can you get the Iris D42 out of mothballs and see if this works for you?
I also have problems with the glass cockpit re. fps.
Have been flying with everything electrical I can click turned on and then power off switch (shift/e). Hits my taget framerate every time, night flying have to depend on map function. As you suggest, it's not the plane, it's the panel, more specifically, the glass gauges.
I searched for panel.cfg stuff, and found 'update_rate=6'. (default is 18mgz)
I put this into every window entry in the panel.cfg and it seemed to help, especially on external views.
Still sucked a bit, then tried 2. Much better on external view and just a bit jerky inside, but then again, on my 3rd beer.
Hope this helps.