When in copy mode (<prefix>[), you can move the cursor around like you
would in vim with the directional keys (hjkl). This works fine until you
want to move up or down through pages and pages of text, such as when
navigating to the top of a long stack trace. One way to get where you need
to be more quickly is by paging up and down.
Hit CTRL-u to page up and CTRL-d to page down.