Many apologies... I spoke too soon before getting more data. Turns out all scripts run faster on previous versions. Users have reported that they favour 6.2 and even some won't use anything newer than 6.17. At first I didn't believe, but after installing clean mirc's with no additional scripts or fish, there were noticable speed differences on small test loops. Even a generic socket read from web sites showed quite a difference. So no need to worry about fish, I imagine there's more to this than I have time to research. But as for mirc, I've enjoyed it from the beginning. It's always nice to take a break from other languages and write scripts in mirc. I'm hoping that once I buy a cpu with more than 4 cores it'll make the difference not so noticable.
Again you haven't posted details of your "tests," so it's impossible for us to look into the script(s) you supposedly tested and what is causing the problem. As Riamus said, mIRC 7.xx is slightly slower than 6.xx but the differences are negligible and you should not see noticeable slowdowns.
In fact, Khaled often re-writes routines so that they're faster than they previously were; you need to get the newest version to gain these benefits.
mIRC is single threaded (other than $dllcall and $comcall) and does not take advantage of multi-core processors so you won't see any improvements. Well, besides the improvements from the load being shared amongst the cores by the OS if you're running a newish OS.