Comment 13 for bug 1675513

IS provided a fix to speed up DNS lookups and the build now completes in around 16 mins (vs original 7 mins)
https://code.launchpad.net/~ubuntu-sdk-team/+snap/ubuntu-app-platform/+build/30005

"The majority of the slowdown was a combination of snapcraft's lack of connection preservation and slow DNS resolution of the builder proxy due to a possible BIND bug. IS has applied a workaround for the BIND problem, fixing much of the latency and most of the build slowdown."

So while the lack of a persistent connection slows initial builds at least they now complete in a reasonable time on LP.