I don't know how much overhead is involved, but I find similar speeds (including that fluctuation between 900 and 3000 kbps over a couple minutes, with speeds gradually settling down to a more-or-less steady 2000 kbps) on my SB when viewed on a computer through the Internet from a distance of about 1,000 miles away from the SB's physical location. And when I am testing viewing within the same LAN as the SB, I typically see the speed ramp up and hold nearly steady at about 8000 kbps. So, its seem clear to me that going through the cable modem and Internet World (instead of staying with the router's own controlled local network) definitely reduces the reported speed significantly. But why such dramatic fluctuation? I assume it's all related to the overall traffic on the Internet on a second-by-second basis and how my specific data packets are being routed from my SB to my computer(s) many, many miles away.
BTW, I find that the speeds that my ISP advertises for download and upload speeds are the MAXIMUM obtained, and not the actual speeds delivered on a day-in, day-out basis. When I was using their "economy contract" with lower advertised speeds, I could see clear evidence in my speed tests of my speeds being much higher, then throttled back after a few seconds to the advertised speeds. Moving from "economy contract" to "standard contract" with the ISP where my SB is physically located eliminated all of the frequent instances of video stuttering and stalling that I had been experiencing. And the observed upload speeds to the Internet when testing that connection increased from 300 kbps to several thousand kbps.