This is standard behaviour in Windows applications. When you first move over an item with a tooltip, there is an initial delay and only then is the tooltip displayed. If you then move to another item with a tooltip, the tooltip is displayed immediately.
That said, I have noticed that some applications, like Firefox and Internet Explorer, display their tooltips slowly all the time. Other applications, such as Notepad++, use the same tooltip display method as mIRC.
It is likely that you were using a very old version of mIRC before that was not using the standard tooltip display delay that it should have been, which was fixed in subsequent versions.