Will the video play in normal speed?
It depends on how powerful your computer is. What are your computer's specs?
If you have a really strong computer, then you'll likely get a smooth 50-60 FPS for your overall Dolphin play. Though the FPS might slightly dip a little when recording, since recording software can get a bit heavy on your CPU usage.
If you have a slightly higher than average (or somewhere close), then you'll possibly get 50-60 FPS with some games. Though the FPS will likely take a huge dip to about 40-50 FPS for those games.
If you have a weak computer, then you shouldn't bother. Invest in getting better computer parts.