If that speedtest is actually indicative of your connection speed, you'd have to be saturating it 24/7 for about 24 days to get 700GB of usage, just pointing it out heh. Is your speed still that slow, and is that the case to multiple different test destinations? telecom.co.nz/speedtest is usually pretty good. It may be something at your end, rather than slingshot, that's causing your speed issues, and its probably worth at least working through the basics before swapping ISP and potentially still being slow. That said, I don't know what throttling Slingshot do for such heavy use, so it may be worth asking them, too.