Several years ago I put a USB hard drive on my development RPI using procedures like this:
I knew it helped, but not really to what extent.
Today, I had to upgrade the RPI from Wheezy to Jessie to Stretch. My SD card based RPIs took 1 long day or two 8 hour days to do the update.
The development RPI running the USB hard drive was upgraded in 1/2 a day, if even that. Clearly the USB system is much faster.