I really have to say, as this doesnt effect all video cards using extened screens (multi monitors) only a few, this truely IMO points to it being video driver specific, whcih means mirc isnt going slow, its the cards drivers (or the card) that isnt going fast. I dont see why any program should have to code around a problem in a subset of possable hardware. I mean that was one of the ideas behind the windows gui, to be able to remove need to have to deal with specific hardware but the programs that are running on it.
If your system responds crapply then dont expect mirc to fix it, else the next person well be demanding that mirc make there connection speed better on there 56k modem.
ps: also rememer should Klahed try to fix this (i dont think he should but thats not my call), he first has to reproduce it, whcih given that he hasnt fixed it so far I assume he hasnt been able to do.