Start with comparison
Test another device in the same room on the same Wi-Fi. If the second device is fast, that strongly suggests the issue is local to the slow device rather than a complete internet outage.
Check signal and band
Move closer to the router and retest. If performance improves sharply, the device may be using a weaker band, have a less capable wireless adapter, or simply be in a poor radio location.
Check background activity
Cloud sync, operating-system updates, game downloads and backup tools can consume bandwidth without being obvious. Pause heavy transfers before comparing speeds.
Check VPN, proxy and DNS
Temporarily disconnect a VPN and verify that no old manual proxy is configured. If raw connectivity is fine but browsing feels delayed, DNS can also be part of the problem.
Check the adapter
Restart the device, review the manufacturer's current Wi-Fi driver, and compare Ethernet if available. If Ethernet is fast while Wi-Fi remains poor, focus on the wireless path rather than the ISP.