You could also, instead of creating a new screen set, just create a new property group in the Clock cell that displays only when the time left is less than 60 seconds. Then set the default property group to display the clock without the seconds, and the new property group to display the clock with the seconds.
The screen set option that tandemrx mentioned could actually be better (harder to maintain if you change other items on the screen, since you may then be forced to make the changes on two screens), but it might run faster.
Other options are to lower the font point size for the clock. And probably the best (although maybe not as aesthetically pleasing) is to lower the resolution of the screen.
Testing 2.5 connected to an LCD TV with 1920x1080 resolution, my laptop slowed a little trying to update the game window, and it's a 2+GHz Core Duo 2. Lowering the resolution will definitely speed IE up.