NetDiag+
Why your ping spikes when the connection is busy
The pattern is always the same: latency is fine until something starts downloading, then it jumps by hundreds of milliseconds for a couple of seconds and settles again. Calls stutter, games rubber-band, and every speed test insists the line is healthy. That is bufferbloat — a queue somewhere on the path holding your packets while it drains a bulk transfer.
Three steps that name the culprit
- Measure idle latency first. With nothing else using the network, ping a stable address for a few seconds and note the median. This is your baseline — without it the second number means nothing.
- Measure again under load. Saturate the line with a download, then with an upload, and keep pinging throughout. The rise over your baseline is the bufferbloat: under 30 ms is healthy, over 200 ms is what breaks calls.
- Read the direction. Latency that explodes while uploading points at the buffer in your own router or modem. Latency that explodes while downloading points at your provider's equipment. The two have completely different fixes.
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Questions people actually ask
My speed test says 500 Mbps — why do I still lag?
Because a speed test measures how much data fits through an empty pipe, not how long one packet waits when the pipe is full. Bandwidth and latency-under-load are different measurements, and only the second predicts whether a video call survives.
Is this my router's fault or my provider's?
The direction tells you. Upload bloat is almost always the buffer in your own router or modem, because that is the device choosing what to send next. Download bloat sits in your provider's equipment, because they choose what to send you.
Why did my provider say everything is fine?
Their tests check the line: signal levels, error counters, throughput. All of those can be perfect while a queue adds 300 ms of delay. You are not being fobbed off — they are measuring something else.
Can I fix it myself?
If the bloat is on upload, usually yes: enable SQM, QoS or fq_codel on your router and cap the upload at roughly 90% of your line speed. That deliberately trades a little bandwidth for a short queue. Download bloat you can only soften by rate-limiting below your line speed.
My phone tests clean but my PC still lags. What now?
Then the network is not the problem and the fault is in that machine. Network adapter drivers are the usual suspect: generic in-box drivers ship with oversized transmit buffers and no queue management, which produces exactly these spikes. A VPN client or security suite can do the same.
Bufferbloat is not the only thing that quietly breaks a healthy-looking connection. Next: how to find your real Path MTU →