But to answer your question, digital to analogue and analogue to digital conversions tend to be so fast that you don't notice. It is more of a convention thing that most VGA devices will display the image as the signal arrives, which means they have almost no latency. DP devices, on the other hand, tend to cache the image, do processing on the entire frame, and only then start the presentation.
As a result, for VGA the latency can be less than the time that it takes to send the entire picture through the wire. For DP, it always is at least one full transmission time of latency.
