It has negligible effect on framerates - more in FSX and P3D than other sims or games, but even that is minimal.
I normally use D3DGear - it has seperate modes for desktop and game recording and can be adjusted in many ways, if desired, for quality, performance effect etc, and can be used for streaming and so-on, if I ever do that (which I doubt).
It’s also currently the only program that can record directly from the Rift output, so I use it for 3D SBS videos, too. The file sizes are much smaller than other recording utes produce, too.
I also occasionally use Movavi recorder, which is what I use when I am using the Rift, but recording in 2D from the monitor. I haven’t noticed much performance impact for that, but I haven’t used it that much. It’s also what I use for editing.