So you’re getting what looks like about 40 Mb/s in London, and a very inconsistent 40-80 Mb/s from New York, right?
Can you try this command to attempt a core snap download?
curl $(curl -H 'X-Ubuntu-Series: 16' https://api.snapcraft.io/api/v1/snaps/details/core | jq '.download_url' -r)
This will spit out a redirect URL. Can you confirm this is an internap URL?
Next, please actually ask curl to follow the redirect:
curl -L $(curl -H 'X-Ubuntu-Series: 16' https://api.snapcraft.io/api/v1/snaps/details/core | jq '.download_url' -r) >/dev/null
and check the speed it reports for the last download (the actual snap; the others are just redirects and metadata).
If you could also try with libreoffice and/or other largish snaps and do a couple attempts for each and get us that data, that’d be appreciated.
FWIW, my own unscientific tests seem to back up your observation up to a point: I got about 40 MB/s (Bytes, not bits; this is what curl reports) for core downloads, over 10 attempts or so. A larger snap e.g. libreoffice, at 351 MB, got very erratic speeds: one attempt got 350 kB/s, another one got 10-11 MB/s, third attempt got 50 MB/s and fourth got about 200 kB/s again; a few subsequent tries saw speeds as high as 80 MB/s. rocketchat-server, at 150 MB size, got similarly erratic download speeds.
What I’m mainly interested in is whether these all come from internap URLs, because we may need to troubleshoot this on the CDN side. These don’t come directly from the store (unless they do 
which is why I’m asking you to confirm you’re getting Internap URLs).
