Gnome uses the Metacity window manager; by default, windows are redrawn while you move and resize them, rather than only when you have finished the operation. There doesn't seem to be a way to turn off this feature, and on some machines this eye-candy is 'expensive'.
To disable it you must use the GConf system: either use the CLI tool gconftool
gconftool-2 -s -t bool /apps/metacity/general/reduced_resources true
Or use the interactive GUI gconf-editor to set the /apps/metacity/general/reduced_resources config variable.
This setting takes effect immediately and is saved automatically so that you only need do it once.