The difficult thing I have when teaching people to ornament Bach correctly is the lightness in their touch. The actual speed of the trills are arbitrary, they have no exact value. I demonstrate this fact to students studying Bach by playing the phrase with a extremely rapid trill, moderate and with a very slow trill or even simply a turn!
When you practice the piece you can neglect the trills first then slowly add it, what however is most important is that you use the ornament to connect the next phrase of music, not make it sound like a chunk of sound just added in. So targeting improvement of your ornamentation with speed is the wrong way to go, what you have to do is try to undertsand how the lightness of ornaments connect the sound, then we learn to be flexible with the notes that connect the sound (in note value and actual number of ornamental notes).