There will be an impact on performance, in that calculating caustics always involves more calculating than not calculating caustics :) So adding them into a scene will have some impact, there is more work to do so it will take longer to do it - caustics will never be "free" in terms of cost. We can't say how much impact as yet though, as one thing about development builds is that they run slower than production builds, and on top of that the code itself is still being refined as well, so it doesn't make sense yet to measure performance until things are further along.
