To balance speed and quality, make multiple mp3's starting from the lowest bit depth and sampling size, up to the highest, and choose the one that is fastest with acceptable sound. Once you auditioned enough, you will have a good idea of the range of bit and sample frequencies that are closer to the ideal.Also, figuring out how to make mp3s sound reasonable is a necessary part of being in this business as you have to upload mp3s to sites like Taxi for submissions and many libraries ask for them for initial review.
You don't have control over how the codecs are programmed however.