In the strictest sense of the words, "slow" and "quick" are supposed to refer to very specific beat values: 2 and 1, respectively (or, for us musician types, that's half or quarter note).
So your teacher is not using slow and quick in the literal sense, but more to express the idea that the first step of the measure should be drawn out slightly.
Since there are only 3 beats, you can't literally have slow-quick-quick (which mathematically adds up to 4), but you can interpret rhythms by stretching certain beats and squeezing others. In other words, you don't necessarily take all three steps exactly right at the top of each beat. Some steps may land slightly behind the beat.
This is similar to the way a jazz musician plays. A jazz musician lays back on the beat in such a way that the melody or rhythm "grooves" with a very human and musical feel. Dancers also stretch and sqeeze beats for a more artistic and musical interpretation.
There's also a matter of logistics... We still have to bow to the almighty laws of physics as they pertain to the human body. Certain constructs of human body movement dictate how fast or slow (in literal time) we can take a particular action. And we have, out of necessity, formed our dance techniques around these rules.
With Viennese, I beleive it's more the physical necessity, rather than the artistic interpretation of the music, that causes us to draw out the 1 and squeeze the 2 and 3.
It takes a certain amount of physical time to prepare and then execute a really good forward driving walk with swinging action. In fact, the Viennese Tempo does not quite allow us all of the time we need to execute that perfect walk. So we take measures to insure that we get as much time as possible, like lowering sooner, not rising as much as we do in other dances, using momentum from previous the step and, as you noted, lengthening the time of the first step and borrowing that time from steps 2 and 3.
The most important thing to understand about this is that you spend all of the necessary time to prepare, execute, and then follow through the forward (or backward) step on count 1. If you do this well, then the timing of the steps you mentioned will naturally result.
I hope this helps.
Sincerely,
Jonathan Atkinson