First thing to do is to change
Qt Code: 
  1. timer->start(10);
To copy to clipboard, switch view to plain text mode 
to
Qt Code: 
  1. timer->start(50);
To copy to clipboard, switch view to plain text mode 
. You won't be able to see 100 frames per second so you can safely limit it to 20 frames per second. You should immediately get a five times increase in speed

Second of all you are murdering your processor data cache by processing data from the last line to the first one. Try reversing the order and you should get significant speedup there. It would also be a good idea to process data in larger chunks instead of single lines. Using a chunk of size equivalent to memory page size in your platform (like 1kB or 4kB) should result in a better speed as well.

All this should already give you much. If that's not enough, we can go further.

One more thing - don't expect miracles when scrolling such a large image. Currently used graphics architectures are not prepared for good scrolling performance (contrary to times where most games involved scrolling the "game board").