Opera Mini is a server based app. When you go to a website, the OM client on your phone sends a message to the Opera server which gets the webpage and interprets it. It then sends just the display content to your phone.
By doing this OM should be fast because it uses the powerfule computing resources availbe at the server. Howeve if the serve is busy, slow, failing that day then OM will display slower on your phone.
Doing all this means that OM's security is far lower than the native browser and since RIM will not let OM onto blackberry.net (I think because it is so insecure) you are forced to use APN / IP (normal internet) for browsing. Any webpage you look at could possible be viewed on the server by someone. Why they would want to do this is beyond me because no-one in their right mind would use OM for anything other than casual browsing. Any banking or financial browsing would obviously be done using the native browser for these reasons.
Even though it may be faster, I would not have OM on my phone if you paid me to do so because it is so insecure.