LSD batteries (such as Eneloops or the Magic-Flight stock batteries) can be charged in fast chargers, as long as they are not too fast. Charging an LSD battery in anything less than an hour is probably too fast. A charger that charges them in about one or two hours is about ideal. Furthermore, the performance of a battery is defined largely by the quality of the charger.
Low self discharge (LSD) NiMH batteries have a chemistry and physical constitution that differs slightly from standard rechargeable batteries. As a result, LSD batteries are more sensitive to overheating than the standard NiMH. Overheating a LSD battery will have a more significant impact, in terms of the loss of battery life (number of charge cycles) and the amount of charge that the battery will retain.
Statistically speaking, ultra-fast chargers—chargers that charge a battery in less than an hour—present too high a risk profile in that detecting charge endpoint conditions is not an exact process. LSD batteries will not tolerate overcharging mistakes as well as standard ones (ie, putting an already charged battery into a fast 15 minute charger is bad). As such, it is really not a good idea to put the stock batteries in a 15 minute fast charger more than occasionally. Magic-Flight does not recommend combining any 15 minute charger with LSD batteries except on rare occasion and when in great need.
The charger that Magic-Flight provides is actually a very good fast (though not the fastest) charger. Its maximum charging time is about 4 hours, and is usually noticeably less.