For what it's worth, I've had loads of references get back to me slowly/not at all and I don't think I've ever judged a client for it.

There's not much you can do. Swordfish's suggestion to reach out to your reference and let her know to expect the request is a good one. Mistress Tissa's suggestion to book farther in advance if the domme requires references is also solid. You can also give numerous references in hopes that at least one of them will answer her emails quickly. That's about all your options.

A well-written email that mentions your willingness to give a deposit may also speed things along considerably. A lot of Mistresses use references because they're easier to get than deposits, and will sometimes accept a deposit instead.