If you have a few hundred SMSs on your phone, Conversation slows down the entire Contacts application. That's very, very, very annoying if you just want to launch Contacts to make a phone call.
It's also the reason why I removed Conversation from my N78.
Can this problem be fixed? The best way would be to pull Conversation out of the Contacts app and put it in the Messaging app instead. That's where most people would expect to find their threaded SMSs anyway, because the Messaging application is called "Messaging" for a reason.
If you have a good reason to keep Conversation in the Contacts app, at least fix it in such a way that Conversation never slows down loading of the basic phonebook. Believe it or not, some people still use their phones to make phone calls.