“No” in answer to “Is updating Ubuntu Snappy Core painful”.
I having difficulty decyphering what the rest of your actual question is.
Are you implying
snap refresh is slow for you due to your slow internet connection, or that the store downloads are slow while other things are fast?
Bear in mind snap refreshes automatically in the background. So as long as your system has a connection to the Internet, the refresh will happen each time the system checks. You can of course force refresh or postpone the refresh.
Also note that Ubuntu Core is more tailored for single-use devices, like digital signage, routers, gateways etc. Not so much for general purpose computing like laptops/servers.