The "Update Frequency" should limit how often your hardware is called most of the time. However you really need to handle threading and throttling in the ASCOM driver like Chris is recommending. It's not all that difficult to implement. Minimally you should throw a read/write locking mechanism (mutex) around the serial communications to keep things atomic (this will keep your serial communication thread safe). if you need to throttle just create a caching object and returned cached values when LastReadTime + Throttling < CurrentTime
I've done this to a handful of drivers. It really only takes about 15 minutes to make them work with a highly concurrent multi-threaded application like SGP.
Thanks,
Jared