If you tried to record at 60FPS and your game was running at 6-18FPS you'd end up with a recording of a 6-18FPS program, in 60FPS. Putting it simply, it would record your computer at 60FPS but you won't really see a difference in the end result because the viewer itself is chugging at a lower FPS.
If you managed to get up to 60FPS more consistently then yeah 60FPS might be a good thing to consider. You'd effectively have a file with the smoothness you usually see in 60+FPS scenarios. But if like the scenario you gave, you're running inworld at 15FPS it's kinda really not gonna be worth it. You're still gonna end up with a chunkier file, that runs as fast as you see inworld, pretty much.