Mine appears to be going slower on AT&T using imessage to send pictures. Doesn't seem to matter what size. image from the net, image taken with camera, or screen shot. They all seem slower than what I remembered using my 4s, but I also haven't used the 4s since the iP5 announcement when I sold it.
It always goes.
The network automaticly shrinks it and resizes it before its send out to the recipients device. So the quality you have on your phone is not what will arrive at the other persons phone.
To get the actual size and quality you should use imessage or email.