want to speed it up use a small handheld screw gun, unthread each nipple about 3 turns, then tape the two rims together. Unthread a spoke with the screw gun, set the new nipple and thread till there's about 2 threads still showing. You can do this all the way around. Note, if you could see any part of a thread on the original wheel you'll need to adjust the visible threads. Something like the cyclus nipple driver
https://www.bike24.com/p2242915.html will also speed up the whole process. Var and a couple other brands make similar and they cost 15-30 each but totally worth the cost if you do this often enough. With this and a bit of practice you can build a wheel in half an hour.