Commonly, WAN's simply connect LAN's with a single connection. 15.3 LAN's connect computers that are geographically closer, for example in an office. WAN's connect computers that are far apart, for example different research labs.
Connections are typically slower and less reliable.
The layers have a lot of overlap so that you can save overhead by combining various stages. The problem is that code becomes more complex and harder to change if the protocol ever changes. 15.6 Increased system speed means that computers can transfer packets faster, which may mean more data is transmitted. As more data is transferred, performance may decrease due to collisions. If a large number of data is being sent, collisions will reduce performance.