[AB] Prime Time slowdowns. Mostly.

Hello folks! I'm trying to understand the rather dramatic download speed reduction I'm seeing during prime time (starting around 17:00 and returning to normal around midnight). This is on an older, grandfathered BB100 plan. I'm in Medicine Hat, Alberta. I don't want to spam the forum with images, so I hope you'll be OK with me just typing the values. These are all done around 18:30, local time. speedtest.shaw.ca -> 8.52 Mbps down / 10.84 Mbps up speedtest.net Calgary Server, Shaw Hosting -> 11.3 / 9.93 Calgary Server, Telus Hosting -> 106.59 / 10.79 Edmonton Server, Shaw Hosting -> 106.76 / 10.9 Edmonton Server, Telus Hosting -> 106.55 / 10.81 Victoria Server, Shaw Hosting -> 5.07 / 10.66 Vancouver Server, Shaw Hosting -> 106.45 / 10.82 Which I find... puzzling. The consistent time-of-day changes suggests a saturation of some kind, but just the same, this doesn't seem like "a saturated node", or, at least, not as I understand the term. Perhaps my understanding is confused. Any suggestions on how to interpret this? Presumably it is nothing directly local (that is, in the house), since the speed drop is correlated with the time of day, in addition to being wonderful with some servers. And same for the node -- surely that would affect all servers equally, would it not? Likewise with the signal to the modem. I'm most assuredly not an expert, but I'm having a really difficult time convincing myself that it is possible for the signal to the modem to affect speeds from one location, but not another. So where does that leave me? Other than confused.