I've been contacted by BetFair with a polite request to set our minimum refresh rate to 0.2 sec.
It turned out that this is one of the requirements to all software that works through the API:
Application Security CertificationYour application should not make any more than five market prices requests, per market in any one second. It appears that despite our warnings an increased number of customers started complaining about exceeding the request limit and thus being liable to charges. Obviously this has something to do with setting the refresh rate to 0.1 sec - at this rate it is sometimes impossible to maintain efficient request limit control.
So I'm very sorry, but we'll be putting the min. refresh rate to 0.2 sec in the next software update.