Time lag to clear image

Hi,
Have a script function which changes the style of an image as display:none after an activity is complete. There seems to be peculiar time lag of severak seconds on some machines to do so. Could any one suggest something abt this ? Does it have any thing to do with machine configuration ?

The time is dependent, among other things, on the CPU frequences, but it should be rather a problem of milliseconds difference for a simple javascript code. Maybe it depends on which activity you are talking about...