Slow Wi-Fi can come from the internet connection, the wireless link, congestion, interference, router placement, device limitations or a remote service. Isolating the layer is faster than changing settings randomly.
Compare paths
Test near the router, then farther away. If possible compare Wi-Fi with Ethernet.
Check contention
Pause large downloads, cloud backups and uploads. Re-test latency and throughput.
Inspect placement
Keep the access point elevated and open, not hidden behind dense walls, metal furniture or appliances.
Know the browser limit
This website cannot read your Wi-Fi channel, WPA mode or access-point signal strength on iPhone Safari.