Frankly, on my system, I find ALS actually improves my frame rate. Rembrandt, however, absolutely cripples many systems - it is most certainly not efficient.
The
urban and
overlay (overlay since they introduced volumetric grass) shaders also tend to be problematic.
Another possibility is that you might be running FlightGear without actually selecting your graphics card?
On Windows, find the Nividia Control Panel by right clicking on the desktop. Then, go to
Manage 3D Settings. In
Program Settings select fgfs.exe and ensure it is using your Nividia graphics card.
The next possibility is that you might be running out of memory. Running detailed scenery, with vegetation, random buildings, etc. could result in this.
If you try running FlightGear with the default renderer (not ALS or Rembrandt), default (basic) weather, no shaders, no vegetation, etc, and still get low performance, there is unfortunately most likely a problem with your system. It should be capably of it, so I'm not sure what's going on if so...