So I've figured out my speed issue, and maybe this will help any other guys who are having a similar issue.
I was getting very slow (3mb/s) download/streaming pretty much no matter what I tried: with a VPN, without a VPN, trying a bunch of different SLR servers and various tests with Tony from SLR Support, etc. Nothing was improving it beyond the very slow speed, which was a pretty recent issue that came up in the past few weeks.
Just for fun to absolutely guarantee it wasn't an ISP throttling issue (even though I was already sure it wasn't), I downloaded a different VPN than the ones I'd tried before. Bingo, it worked.
That was literally the issue: it was my ISP throttling, but the problem was that the VPN services I had tried using to test this initially were various free or "sample" versions of their VPN, and had severely limited options as far as which servers you could select to use. So the servers that the free VPN's defaulted to were poor ones, and gave me entirely inaccurate info and slow speeds as a result. With an actual full VPN, I've tested a handful of the 100+ server options available with it, and so far every single one has been at the very lowest 3x as fast with MB/s download speeds from SLR.
TL;DR: even though I was sure it wasn't an ISP throttling issue, it was. And using an actual full VPN solved it.