Finding the constraint.
WHAT TO ESTABLISH
Slow for everyone or one device? Everything or one destination? All the time or at certain times? Wired as well as wireless?
ONE DEVICE ONLY
The device, or its wireless link.
ALL DEVICES, WIRELESS ONLY
The wireless. Test on a cable to confirm.
ALL DEVICES, WIRED TOO
The connection, the router, or something saturating it.
WHAT SATURATES A CONNECTION
Backups running Large uploads or downloads Video streaming Software updates across many devices
WHAT TO CHECK
What is actually using the connection.
Many routers show per-device usage.
CERTAIN TIMES ONLY
Contention, or something scheduled.
ONE DESTINATION ONLY
A problem on the path or at that service, not your connection.
Trace the route to see where.
WHAT TO TRY
Restart the router Test on a cable Test with other devices off
WHAT TO RECORD BEFORE CONTACTING A PROVIDER
Results, times, and what you have ruled out.