While MuPDF is engineered to be as fast as possible at interpreting page contents, there is inevitably some overhead in converting from the documents native format to the stream of graphical operations (calls over the fz_device interface).
If you are planning to redraw the same page several times (perhaps because you are panning and zooming around a page in a viewer), then it can be advantageous to use a display List.
A display list is simply a way of packaging up a stream of graphical operations so that they can be efficiently played back, possibly with different transforms or clip rectangles.
Display lists are optimised to use as little memory as possible, but clearly are (typically) a greater user of memory than just reinterpreting the file. The big advantage of display lists, other than their speed, is that they can safely be played back without touching the underlying file. This means they can be used in other threads without having to worry about contention.
Display lists are implemented within using MuPDF using the fz_display_list type.