Long delay, and the problem remains. My ISP has gotten back to me with a bit of info as stated below, though:
"We've ran a bunch of packet captures and analysis, and approximately 40-45% of the TCP packets coming from your system are being retransmitted, this means that the higher the latency is between 2 endpoints, the lower the throughput (at a dramatic factor), which is why we can test to the tower/headend with minimal issues. We can't reproduce/see behavior to any other device on that network segment.
At this point, we'd need to look at putting some type of a managed switch/router to run some tests from inside/outside of your network; the issue there is that nobody, including the hardware vendor support, can identify anything at fault on our network that's causing the retransmissions."
From what I've read, normal max TCP retransmissions should be 2% before performance is noticeably impacted, so I could see this causing the issues. However, since speeds to their internal ISP location are fine, it would seem like an issue with equipment beyond that point either not sending back acknowledgements or dropping packets due to buffer overruns caused by network congestion. Either way, I can't see how putting a managed switch in my network would lead to any solution. I asked if it could be an MTU issue, but I highly doubt it.
Anyway, with this new info, can anyone smarter than me offer any insight?