Lots of "L" words kept popping into my head and I was having trouble choosing just one as they are all important.

L is for Latency - Lori and Joe have previously written about latency so I won't rehash what has already been said.

L is for Load Balancing - There are many types of load balancing that are important with acceleration;  server or local load balancing to ensure that the requests are evenly distributed across all available servers,  link load balancing to choose the best performing link for traffic and maintain high availability, and global load balancing to direct users to the closest data centre (particularly important when deploying symmetric web acceleration).

L is for Link Speed - Many times application performance is evaluated only from high broadband connections or on the LAN (hey another "L" word) which is not representative of the true user experience.  Application performance can vary significantly depending on the connection speed a page that takes 5 seconds on a high broadband connection may take anywhere from 30 - 70 seconds as this report shows.  While there may not be many people using dial-up connections these days the number of people using mobile devices to browse the Internet is steadily increasing.  The performance over mobile devices may show similar if not worse performance depending on the latency.