"Opera mini is a proxy browser, transferring webpages by Opera server. It is not an dependant HEML browser translating and executing procedure by itself." it is the official statement about Opera Mini. When it comes to speed, it is obvious one of the greatest features of Opera Mini. Because Apple adopt to transmit the users' required page to Opera's server and then turn back to users' Opera Mini browser after be compressed. While this method indeed makes the pictures change quickly.
I never tested on Opera Mini but I currently using latest version 10.51 and it is incredible in speed. Opera may be the fastest compared to Firefox and Chrome but not sure with Safari.