I've no way to currently check if my assumption is correct -- but here it goes anyway:
I suspect that
evince cannot directly render and display PostScript on screen. Maybe it can only do so for PDF. So it may use a trick for PostScript files: behind you back it converts them to a temporary PDF file and display that one instead.
Try to install the (very old-fashioned)
gv PostScript viewer on Ubuntu:
sudo apt-get install gv
and see if it is any faster. (
gv also uses PostScript for rendering, but doesn't convert to PDF.) Or you use Ghostscript directly
But don't forget one thing: exactly because PostScript is a programming language, you can design PostScript files which are rather short in Bytes, but make the interpreter go through a loop with thousands of iterations (for example to compute + render a fractal) before it displays the file content on screen.
PDF doesn't have 'loops' and computations like that...