Setting the fastScroll resource to true allows xterm to emulate the behavior of later terminal emulators, which discard screen updates for speed purposes ("speed" being measured by "how long does it take to cat a large file to the terminal in its entirety").

Before this feature, xterm was considered the slowest terminal emulator because it updated the screen with each write, the way real terminals do. But now you can turn on fastScroll and amaze your friends with fast cat times.

Glamor was slow as hell. It got better when Keith Packard stopped calling glFlush() after every draw call.