Any way to tell if it's a throttle vs poor hardware/connection?

Plug your computer straight into the modem and run speed test.

LEASTBAY is trying to eliminate your router as the source of flakiness. Run several speed tests with a computer plugged into the router (and router plugged into modem), then run several more with the computer plugged straight into the modem (where the router cable would normally be).

If it's significantly slower and/or less consistent going through the router, that would point to the router being the issue. If the results are slow and/or inconsistent either way, then the problem is probably with your modem or ISP.