Slow artifact publishing?

I have a build machine in the US publishing a 15MB zipped artifact to a server in New Zealand, but it's taking much longer than expected. I generally see transfer speeds of around 500KB/s over HTTP or Windows shares, but the artifact is taking around 5-10 minutes to publish, which is significantly longer than the <1 minute that I would expect it to publish in. Turning on debug output does not seem to provide any more particularly helpful information. Why is this one artifact taking so long to publish?