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Things get tweaked with each update, as we add new features. So I think even the same settings will sometimes "drop" fps after an update.
However, a given rendering setting in X-Plane will impact *either* the CPU or the GPU (i.e., the graphics card). Your overall framerate will be limited by whichever of those two is most heavily loaded.
In the ideal case, your rendering settings would be perfectly tuned to keep both your CPU and GPU fully loaded at whatever frame rate target you're trying to hit (often 30 frames per second).
But, if one component is overloaded---for instance, if the CPU-affecting settings are turned up much higher than the GPU-affecting settings---that component will become the limiting factor; it will "bottleneck" the rest of the system. In this case---with the CPU bottlenecking the framerate---you could swap in a hypothetical infinitely powerful graphics card and see little to no performance improvement... because it wasn't the GPU that was limiting your framerate in the first place.
So, my recommendation would be: check and make sure you're not bottlenecked on your CPU. The rendering options portions of the manual should give you an indication of which settings affect the CPU versus GPU. Likewise, the section on tuning the rendering options for best performance may be of use---it will walk through turning down all the rendering options to start with, then slowly turning things up until you find the best setting for your machine.
The response from jroberts is from the X-plane (Laminar Research) expert on the product.
I am no expert on X-plane, however, I came across the following problem raised in this forum as well. Others in this forum, as listed in recent postings are also experiencing frame rate losses with a Win10 installation and recent upgrades.
As you are running Windows 10 the solution to your problem may be as discussed in this same forum at http://questions.x-plane.com/3673/performance-windows-updates-kb3176936-kb3176938-kb3189866
Hope this also helps as another possible solution to what jroberts has suggested.
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