The Transmission Time is intended to keep track of the time spent by the browser while consuming the data in the transmission channel between the web server and the browser.
This time is expected to be 0 most of the time, but can increase in occasions when the amount of data (say a very very big table) is sent by the web server.
It’s intention is not to measure how long it takes to the data to go over the network. But the time the browser might held up a connection open to the web server while consuming this data.
In most cases this time is zero, as soon as the data gets to the client the connection is done, but on occasions, particularly when a very big html table is sent to the client, the client keeps this open, preventing the web server from sending more data to the client.
