I haven't measured my typing speed pre/post, but I recall it being pretty easy to switch, and within a few days(?), my typing speed didn't feel like it was slowing me down. If you already touch-type 100% "correctly", all the keys are in the same places on each hand, so it's very easy--your hand is just in a new location, doing the same thing it's always done. I had a few keys where the "wrong" index finger was in the habit of reaching across the centerline to type it, so I had to get used to hitting those keys with the correct index finger, but that's about it.
Once you get used to split, it's no problem to switch back and forth-- I use both my split keyboard and my built-in laptop keyboard without problems.