Most web companies have very high speed Internet connections. This may improve productivity for those that develop on remote, publicly accessible servers, but it can also prevent the development team from appreciating the impact of the design and front-end coding on the site performance for the majority of site visitors.
The easiest way to determine the estimated download time is to use YSlow. Take the number of bytes required to display an uncached page (or unprimed cache) and divide it by the estimated connection speeds. http://websiteoptimization.com is an excellent resource, because it calculates page load/display time for you, as well as illustrating all the page components. It may be affected by caching, so review the report carefully. You can use Google analytics (or any similar service) to see the connection speeds.
Low-tech methods include allowing employees to work from home. DSL is one of the very common access methods, and will show performance issues much better than cable or other very-high-speed paths.
The above link can also be used to understand the time required for the page load and display process.