🚀 go-pugleaf

I wrote a little hack (for viewing GIF files) that keeps a large image
in memory, and bitblt's portions of it to the real screen memory.  Which
part of the image gets blt'ed depends on mouse coordinates, ie by waving
the mouse around, you can move the screen around on the image.  All this
stuff works fine, but for one problem:  there's a fair amount of hash
displayed while the image is moving.  Needless to say, it's somewhat
difficult to see exactly what's hashed, as it flickers quite a bit; my
uncalibrated eyeballs tell me it persists for no more than one scan of
the screen, but that might not be right.

The size of the transfer is screen-size; ie 320 * 200 * 4 bits, or 32000
bytes.  I'm running on a Mega-4, with a blitter.  I expected the blitter
to be able to do things faster than it appears to be doing; even
assuming that it's only doing 16 bits at a time, and that each 16-bit
thing takes 250 nanosec (125 for read and 125 for write), that's still
only .008 seconds, considerably less than one screen refresh time.

I've tried sticking Vsync()'s in strategic places, but it seems to make
no difference.  

What am I missing?  Am I assuming that the blitter's faster than it is?
Is my arithmetic off?  Does Vsync not do what I thought it did?  Is
there something else going on that I don't know about?  Any ideas/hints
appreciated.