Read the statement by Michael Teeuw here.
Slow 'npm install'
The npm install command takes as long as 15-20 minutes and will max the load to about 6-10 with little to no background tasks. Even a fresh reboot does not affect the time. I know its just a wee-raspberry pi 3 but dang that is slow. My Pi shows about a 50C temp with a small heatsink on it with a plastic vented case. Added bonus is sits on top of a server that has a fan blowing moderately ‘cool’ (30-40C) air at high speed. How long should I expect the npm install to take?