4.1 VLAN Basics
4.1.6 VLANs vs. network latency from routers
In the legacy network shown in the Figure, accounting users on the two segments have to cross the engineering segment to transfer any data. The frames have to pass through two routers. Software-based routers tend to be slower than other internetworking products such as a Layer 2 bridge or switch. As a frame passes through a router, the router introduces latency to the network. Latency constitutes the amount of time necessary to transport a frame from the source port to the destination port. Every router that the frame transits increases the end-to-end latency. Further, every congested segment that a frame must cross increases latency.

By moving all the accounting users into one VLAN, the need to cross through multiple routers and segments is eliminated. This reduces latency in a network that will improve performance for your users, especially if they use a send-acknowledge protocol such as TCP. Send-acknowledge protocols do not send more data until an acknowledgement is received referencing the previous data. Network latency dramatically reduces the effective throughput for send-acknowledge protocols. If you can eliminate the need for user traffic to pass through a router by placing users in the same VLAN, you can eliminate cumulative router latency. If frames must pass through routers, enabling Layer 3 switching will significantly reduce router transit latencies as well.