Commands for Diagnosing Slow Internet

Separate layers

Users say “slow internet” for DNS stalls, high RTT, low throughput, or jittery video calls—different causes. Work top-down: resolver → first hop → path → remote service → Wi-Fi vs wired.

Command playbook

Symptom hint	Try	Reads
Pages hang before loading	`dig +stats example.com @resolver`	Query time vs server RTT
Everything feels laggy	`mtr --report -c 100 target`	Loss + latency shape
HTTPS feels slow	`curl -o /dev/null -s -w '%{time_namelookup} %{time_connect} %{time_starttransfer} %{time_total}\n' URL`	Where time accumulates
Throughput only	`iperf3 -c server -R`	Single vs multi stream limits

Bufferbloat signal

If latency spikes only while uploading, suspect saturated uplink queues—run the same ping during an iperf3 upload. Smart queue management (CAKE, fq_codel) on the router often fixes it.

Frequently Asked Questions

Q.Why is Ethernet fast but Wi-Fi slow on the same circuit?

Airtime contention, bad RSSI, band steering, or AP firmware—do not blame the ISP until a wired baseline matches expectations.

Q.What curl timing numbers indicate DNS vs TLS slowness?

High time_namelookup points to DNS. Large gap between time_connect and time_starttransfer may be TLS or server think time—compare multiple URLs on the same host.

Q.Should I trust speedtest.net alone?

Use it as one datapoint; flash-heavy clients hide bufferbloat. Pair with iperf3 to your own VPS and mtr to the same host for reproducible baselines.

Q.Why does slowness happen only during video calls?

UDP realtime traffic suffers from jitter and uplink saturation. Check simultaneous uploads (cloud backup) and enable SQM on the router if latency spikes correlate.

Q.How many parallel iperf3 streams should I run?

Start with `-P 4` or `-P 8` on high-BDP links; single-stream TCP may underfill modern broadband. Document flags so retests match.

Q.Does corporate VPN always add latency?

Expect extra RTT to the concentrator and possible hairpin for split-DNS. Compare traceroute with VPN on/off to the same destination.

Q.Why does only one website crawl while others fly?

Likely remote CDN edge, SNI filtering, or broken IPv6 path—test `curl -4` vs `curl -6` and alternate DNS resolvers before blaming local Wi-Fi.

Q.What baseline should I save after fixing an issue?

Store mtr report, curl -w line, iperf3 summary, interface stats, and router firmware version with timestamps so regressions are obvious.