When things work but slowly.
WHAT TO ESTABLISH
Everything or one service? All devices or one? Wired as well as wireless? Consistently or at certain times?
ONE SERVICE ONLY
The problem is that service or the path to it, not your network.
Trace to it and see where delay appears.
ALL SERVICES, ONE DEVICE
The device or its link.
ALL DEVICES, WIRELESS ONLY
The wireless.
ALL DEVICES, WIRED TOO
The connection, the router, or saturation.
WHAT TO MEASURE
Latency as well as speed Packet loss
WHY
Loss and latency degrade experience more than raw speed.
WHAT TO CHECK ON THE ROUTER
Per-device usage Whether it is overloaded Uptime, and whether a restart helps
WHAT TO CHECK FOR A WEBSITE SPECIFICALLY
Whether the server is slow to respond, or the page is heavy.
Those are different problems.
WHAT TO RULE OUT FIRST
Saturation, which is the most common cause.