You're probably using some script or DLL that is causing the slowdown.
What is the result of typing the following in mIRC:
//echo -a $md5($mircexe,2) $script(0) $dll(0) $com(0)
On a sidenote, what is it about the new version that you "can't stand"? Is it the many bug fixes? All the new features? Better scripting support?