Is triple buffering really a free performance boost?

At 55 FPS, one frame is a bit under 20ms. So it's not quite free, but it's almost always well worth the cost.

At 15 FPS, though, it can be pretty noticeable. (BTW, this is a POS laptop. Most 3D games run about that fast for me. Some even slower.)

Some people notice the lag, especially at lower frame rates, and sometimes triple buffering doesn't work quite as expected. (In OpenGL, in particular, something that's not supported well by the hardware can fall back to (potentially very slow) software emulation. Shouldn't be much of an issue these days, but eh.) So most programs that use triple buffering also include an option to disable it. You may want to consider doing the same.

A quote from the excellent "Real-Time Rendering" book (http://www.realtimerendering.com/) by Akenine-Möller and others, page 835, which pretty much sums up what you have already suggested in the answer to your own question, considering lag:

"Triple buffering has one major advantage over double buffering. Using it, the system can access the pending buffer while waiting for the vertical retrace. With double buffering, a swap can stall the graphics pipeline.[...]The drawback of triple buffering is that the latency increases up to one entire frame. This increase delays the reaction to user inputs, such as keystrokes and mouse or joystick moves."