How are you testing XP/Vista timings? Are you dual booting? Using a virtual machine? Different computers? I can only hope you're taking into account any hardware or cpu usage differences between the two, if you're not dual booting.
It took 5 seconds to minimize (with ESC) 50 windows on my Vista64 machine. I've never noticed any mIRC-related performance issues with Vista before in the 2+ years I've been using it, so I'm skeptical about these claims.
