My main concern about shortening the conversion time further isn't exactly about the risk of price manipulation. I am most concerned that it modifies normal market-based pricing mechanisms for Hive. For example, in a normal market, if you buy enough of a fixed quantity good, you can expect the price to go up.

In the case of HBD, it provides another mechanism for acquiring Hive: instead of buying Hive, you can acquire a lot of HBD, wait for a short term market drop, then convert it to Hive during the time Hive is at a low price. This doesn't apply the same amount of positive pricing pressure that acquiring Hive on the market at that price would likely apply. Instead, this mechanism dilutes the supply of Hive itself. There's nothing outright wrong about this different method of acquiring Hive or the potential inflation mechanism it creates, but it does change the pricing and supply dynamics somewhat, and I don't think we should push it too far.