A website sells more if its speed is good. A slow website will loose customers as present customers do not have time or patience to wait for slow page loading.
With reference to a site built on Magento, speed related steps are bigger concern. Magento uses EAV concept which allows and gives Magento lots of flexibility but this also means in lots of DB tables. Imagine one product data going into more then 40 tables.
Magento team was aware of this shortcoming so they have added indexing and flat table concept. Also they have a good caching concept and for enterprise version. They have full page caching.
Magento development and implementation team should know how to use these concept with proper configuration, standard template, customization with FPC and cache usage in mind.
Also at HTML render make sure that time is less. This can be achieved by reducing number of request (Use of image sprite, merge .js files,merge .css files)
Also minimize 3rd party request (This needs DNS resolution) and if needed make sure these are called after document has been downloaded.
A well optimized site should on dedicated then 3 seconds for complete download (less than 0.8 second to load HTML)
Contact us and we will give tentative estimate that your site page should need to load.
Use Google page speed tool or yslow to check Magento Site Speed.
Please check our Main Service Page of Magento Speed Optimization