With your aircraft cockpit screen up and ready to fly, pull down your View menu (2nd from left) and select Rendering Options.
In that dialog box, the shader control (slider) is in the upper left quadrant. Make sure it is all the way to the left. It will say something about Performance vs Quality, and the number next to the slider will go to zero.
Also, if you haven't already, off of the View menu again, select the Display Options and check the top two boxes, Frame Spacing and Frame Rate. The Frame Spacing number will appear in the lower left corner of your screen as some number of mili-seconds (ms) and the Frame Rate will appear in the lower right corner as a number of frames per second (fps).
Once your cockpit appears, I find it often takes as much as 60 seconds before the Frame Spacing settles down to a reasonable and fairly consistent number, and the Frame Rate goes much above 1. This is because even though FG is showing you the cockpit, it is still very busy setting up scenery where you cannot currently see it. You may see the Frame Spacing jumping between 20 ms and 2000 ms (2 whole seconds per frame. Very un-flyable!). Once FG gets everything set up, it can give you its full attention. Your clue is that your Frame Spacing becomes very consistent at a fairly low number. Anything that stays below about 70 ms will give you flyable performance.