Got my first mechanical keyboard recently (since I was a kid, anyway). Opened the box, plugged it in, and spent about the next 20 minutes doing typing tests. Based on casual observations (I didn't make a spreadsheet project out of it), my speed improved 5-10%. So pretty minor.
On the other hand, I'm at work now, I'm typing noticeably worse on the cheap keyboard plugged into the PC I'm on. It feels off after a week with the Unicomp. So I've probably screwed myself for nearly all the keyboards populating the world.