Inside Out Model
One approach to diagnosing network bottlenecks and problems is to go inside-out, from the original device encountering a network issues, then up and out to broader areas of the network:
The Device
The Device's Network Card ("NIC Card")
Nearby Wireless Access Points
Building Construction
Building Wiring / Internal Wired LAN
Proxy Server and Filter
"Last Mile" / ISP
Internet and Destination/Remote Services