Basically, while the browser and file system are preparing to download the file, it starts transferring in advance. Then when the progress meter appears, it "zooms forward" a little bit, representing the part that's been downloaded in the background.
30-50 kb/s isn't very fast though, what's your connection rated at?
Nitpick: "p" means "pico", so "kbps" would be "kilobit(s) pico second(s)". For "kilobits per second" you want "kb/s" (heaps of people make this mistake so I thought I'd better point it out) 🙂