IE might wait until it has all of the data downloaded/buffered before rendering it, whereas other browsers (Mozilla, for example) might start rendering as soon as they have a set amount (1,024 characters for example).
Without more information (a link would be great), I really don't know what else to say. It could be that at peak times, your server (MySQL, webserver, who knows) could be running slowly, bandwidth is peaking, who knows.