The reason is that your loop is displaying something mostly, and is performing differently on peoples machines is due to how there machines are at that moment.
(1) Increase the loop to a more noticable amount using 10,000 instead of 1000
(2) maximize mirc and maximize the current window then /999test and record result
(3) now make the current window one quater of the maximized size (1/2 width and 1/2 height) then /999test, compare the results (this one well be quicker)
Reason : you are displaying stuff inside your loop, this means that the Windows OS is instructed to redraw a window everytime you go through the loop, the less to redraw the faster.
Other Reasons : Windows is a mutithreaded & mutitasking (well almost) OS, so if you switch windows to another application, the new current application recieves a greater % of the CPU time, so mirc gets less meaning it takes longer to complete. (this is dependent on how windows is setup but most are setup like this)
* Thats the simplest I can explain it, it can get alot more complexe of course