I installed a Samsung 850 Evo 250GB SSD, cloned the OS across from the emmc and then formatted the emmc. Startup from emmc was about 30 seconds, and from SSD was just over 20. The SSD read/write speeds were about 2.5x those of the emmc.
All was fine for a couple of weeks until I started having a ‘Preparing Automatic Repair’ loop problem. No Window’s self-diagnosis tools or resets or system restores could fix it, although disconnecting the battery and re-attaching often cleared it once, until the next time it was shut down / started up again.
Eventually I just had to remove the SSD, fresh install Windows onto emmc, reinstall SSD to get the thing stable. At that point I decided from now on I’ll wait the extra 10 seconds boot time each time rather than risking another X number of hours trying to fix it and installing windows etc. I can still put most programs onto the SSD at installation so they run nice and quickly.