imagedata - why is it so slow ?

[jbv]

I am working on an old stack (built with
MC 2.4.1) in which I make extensive use
of commands such as :
set imagedata of img 1 to imagedata of img 2

most of my images are 1024 x 768.

when run with MC 2.4.1 everything used
to (and still) work at lightspeed (on any platform),
but in Rev 2.5 each command line takes at least
2 to 3 seconds (on any platform)...

Any clue ? Thanks,
JB

PS : I am somewhat forced to convert that stack
to Rev, because I'm also playing QT movies, and
under MC those aren't as clean as in Rev...