Start with a wired test
If possible, connect one device directly to the router and run several tests while large downloads and backups are paused. A good wired result with poor Wi-Fi results points towards local wireless conditions rather than the broadband service itself.
Check for local causes
Look for cloud backups, operating-system updates, game downloads, security cameras and other devices using bandwidth. Restarting equipment can clear temporary faults, but repeated problems usually need a more specific diagnosis.
Compare times and devices
Test at different times of day and on more than one capable device. If every device slows down mainly during busy evening periods, congestion may be involved. If only one device is affected, focus on that device, its Wi-Fi adapter or software.
Look beyond download speed
High latency or jitter can make a connection feel slow even when throughput is good. Upload saturation can also delay ordinary browsing because acknowledgements and interactive traffic must share the upstream queue.