This is the single most useful test you can run. Keep a ping going, then start a big download. If the line stays flat, your slowness is elsewhere. If it balloons the moment traffic starts, that is bufferbloat, and no extra bandwidth will fix it. Start a download and watch.
RTT (ms)time →20 ms
download running
idle line ≈ 20 ms
Idle and healthy. Round-trip is flat near 20 ms. Interactive apps (video calls, SSH, gaming) feel instant. Now start a download and watch what an oversized buffer does.