X forwarding involves granting the remote system access to your X socket, which is very powerful. I don’t know for sure that Plan 9’s model avoids this (I’ve only had small chances here and there to play with Plan 9), but given its heavy use of namespacing I suspect it’s at least possible.
Performance-wise X forwarding was always pretty slow for me in a way that Plan 9 seems to avoid, though I’ve gathered it used to be more efficient prior to modern graphical toolkits that want to draw a bunch of bitmaps to the screen. It’s possible they were more evenly matched in the past.
Not sure about Display Postscript, never had a chance to try that one.
Display Postscript was really great, and X11R5 at least was super snappy, predates the Motif and other window managers slightly, I think. But i also would use super fast window managers and usually not be pushing bitmaps all over.
Performance-wise X forwarding was always pretty slow for me in a way that Plan 9 seems to avoid, though I’ve gathered it used to be more efficient prior to modern graphical toolkits that want to draw a bunch of bitmaps to the screen. It’s possible they were more evenly matched in the past.
Not sure about Display Postscript, never had a chance to try that one.