You can easily practice speed unclipping in a safe environment, such as your yard or a park. You'll probably tip over now and then so wear your helmet and possibly knee & elbow pads. Definitely leave your ego at home.
Really good practice would be to trackstand or do some ultra-slow maneuevering and then unclip when you need to put a foot down. You will get fast at it and you will gain balance which you'll need for slow sections on trails.
However it is naive to think that you'd ever be exactly as fast at unclipping as simply putting your foot down from a platform pedal. It just isn't true. But can you be fast enough? Sure.
What I have found when riding downhill while clipped in is that not only do you have to get out fast, you need to get back in very fast as well, so that you don't fly off the pedal and so that you can be ready for the next transition. Left corner to right, right to jump, jump to left corner, corner to rock garden. Obviously if you're very skilled on your bike and you know the traction limits of it, the terrain, and your tires, then you do all that clipped in and look sweet doing it. Normal people who aren't being paid to ride probably need to unclip and hover a foot, or actually put it down somewhere in that mix.
This applies to any sort of going fast, not just gravity. It takes a lot of practice and skill to ride at the limit and not dump it now and then. Being clipped in just adds another layer of difficulty, but with practice, it's not impossible.
But it also applies to going slow when you might need to put a foot down due to slipping on a rock, wet root, and so on.