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Performance depends on your hardware, you'll typically want to aim for at least 30-60 hz/fps - so something in between (aka 45) should be fine (if steady) - note that your frame spacing latency is separately shown.
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Anything below about 15 frames per second (fps) is close to unusable, or at least, will not be enjoyable. In fact, you'll probably want to aim for 25-35 fps minimum, and a maximum frame spacing (latency) of ~50 ms.
 
Anything below about 15 frames per second (fps) is close to unusable, or at least, will not be enjoyable. In fact, you'll probably want to aim for 25-35 fps minimum, and a maximum frame spacing (latency) of ~50 ms.
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20fps is at the low end of the usability range, it's OK but not great - YMMV. Anything lower than this is pretty much unacceptable, at least for VFR, as things start to get very choppy at 15fps. If 20fps is all our hardware can handle then you will still have a usable sim with acceptable results depending on how high you have your rendering options set. For those with lower end hardware you should probably favor frame rate over eye candy. My personal preference is to never have the frame rate drop below 30fps because things feel smoother and you will have more headroom to handle things that can cause choppiness.
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Older versions of FG (before 1.9) would get choppy if the frame rate got too high (well above the monitor refresh rate) but newer versions don't do this at least not on my hardware and not on older hardware I was running until recently (IE. with versions from 2.0 to 2.4 with a GTX 7900 GPU). As a result now I just let my hardware run at whatever rate it wants to since I now have high end hardware that will handle anything FG throws at it. In complex scenes like around KSFO it will run in the 40s and 50s most of the time and sometimes faster with rendering options up all the way at very high resolution. This is definitely very nice and does not feel laggy even at very high aircraft speeds at very low altitudes (350mph on the deck). In simple scenes (think desert - no trees, no buildings, no clouds, no AI or MP aircraft) I will see frame rates in the 150s to 180s and I don't see any issues with the high frame rate in FG unlike versions before 1.9. Depending on your system setup you may need to throttle the frame rate to prevent choppiness at high frame rates - again YMMV. Most users don't have enough GPU power to be concerned about frame rates getting too high with current FG versions.
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In general the faster the better up to your monitors refresh rate at least. Keep in mind that things like nasal listeners, animations and other parts of the sim run at the video frame (main loop) rate and if your frame rate gets too low these things might not work as well as they should. As an example if your aircraft is one armed with machine guns you may see the guns change rate of fire if your frame rate is too low. This will definitely be the case with the JSBSim P-51D if your frame rates are below 30fps. As more things are decoupled from the main loop this should become less of an issue going forward.
    
Additionally, the more the framerate decreases, the more other features of the simulation will suffer. The current [[FlightGear]] software architecture is such that long rendering times (slow framerates) may prevent other parts of the flight simulator, such as the autopilot, from having sufficient CPU time to respond correctly in the context of the simulation.
 
Additionally, the more the framerate decreases, the more other features of the simulation will suffer. The current [[FlightGear]] software architecture is such that long rendering times (slow framerates) may prevent other parts of the flight simulator, such as the autopilot, from having sufficient CPU time to respond correctly in the context of the simulation.

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