What does TimeStart and TimeEnd signify in the ExecutionLog table

I'm trying to determine where my performance issues are and need some definitions of these fields and what they are indicating in the Execution log file..

I have an applicaiton web server that connects to the reporting servcices web server that generates a pdf.

It takes 5-10 seconds to get the download pdf screen on the application but when i look at the log the difference between start and end time it is 3 seconds. In addition if you add up the time it takes to Process, Render, and Get data that is sub second.

What does TimeStart and TimeEnd signify

thank you