"Extra work always takes extra time, no matter how efficiently you do the extra work" is my quote on this - and it is the truth, caustics are hard calculations, so take quite a lot of processor power to do, and will always add to the render time. From my past experience, the caustics in Corona are fast, compared to other solutions I have tried in the past. This does not mean "only adds 25% to your render time" fast though, as you are adding a great many complex calculations to the scene - in general, 2 to 3 times the original render time can be expected, though I've seen it as low as 1.5 times the render time, and as high as six times the render time. To me, that is very reasonable for adding caustics to the scene.
Of course you can optimize - only enable caustics for light sources that matter (usually, disable them for the environment, and usually most lights don't need to contribute, you'll have 1 to 3 main lights that need to take part in caustics), only enable refractive caustics on materials that matter.
Also, we'll always be looking for ways to optimize too. But, there has to be a realistic expectation too - I doubt caustics will ever be "just adds 10 minutes to your 1 hour render time" (though we shall see!)
