This is best addressed in person with your regular teacher, as there are many possible things- but most importantly is knowing that you will still have to put in time sawing. Kudos on working with your teacher and taking it slow, that is the best way- you'll never get it fast if you can't do it slow.
That said, what errbrev suggests is a great place to start, but also make sure you have fresh hair and good rosin.
Is there any chance that you are squeezing or tightening your bow hold when you are trying to speed things up? Stay relaxed in your hold.
Are you playing in the string or off?
Are you being mindful of the balance point of your bow?
Also, make sure that you are cleanly stopping the notes with your left hand, using your finger tips please! Sometimes too much focus on one hand allows the other to get sloppy!
I hope some of this helps you, but you may find that the cake simply has to spend more time in the oven, so to speak!
Good luck!
Joe