I can confirm that bitcoinqt or bitcoind is not something you may want to use in any larger project. After it has several thousands addresses and running for few months, this official client slows down to the point when it takes few minutes to process every block. In the mean time, it blocks all RPC calls, including ones that wouldn't have to be blocked (such as getdiff or get new address). This slowing down has nothing to do with its internal accounting system (that one works fine, but due to long time blocked RPC calls, it is much less useful than your own accounting system). Why official client slows down so considerably - doing 2 threaded processing for several minutes when new block arrives - that is something devs can explain and maybe even fix in future versions. It is a shame, I would personally trust this client the most, but due to these slow downs, it is useless for us and we will be soon switching over to BitGo.