Very slow plot rendering with X11 on CentOS 5.5

I am connecting from a PC to a Linux system running CentOS
release 5.5 (Final) and it is extremely slow to render plots
to the X11 device.

The Linux timings are just awful, particularly using
X-Win32. Cairo vs. Xlib doesn't seem to matter much.

I have to think it is display rendering load at the displaying 
unit [an X server], or network latencies in getting the detail 
from the producer [an X client] to the displaying unit [an X 
server]

With centos 5 on a local X display [so the X client, and 
the X server do not have to push the content through the X 
fowarding and across the network sockets, but rather can go 
through the lo interface under Linux]:

f <- function(n){

+   for(i in 1:n) qqnorm(rnorm(100))
+ }

system.time(f(20))

user  system elapsed
   0.220   0.028   2.417

which is quite sprightly ;)

-------------------

Then when I run it on the same hardware, but through two SSH 
hops, to, and back from a remote unit in the local subnet, 
things fall apart:

[herrold at centos-5 ~]$ ssh xps400
Last login: Tue Sep 28 11:00:05 2010 from centos-5.first.lan
[herrold at xps400 ~]$ ssh centos-5
herrold at centos-5's password:
Last login: Tue Sep 28 09:13:50 2010
[herrold at centos-5 ~]$ R
  ...

f <- function(n){

+   for(i in 1:n) qqnorm(rnorm(100))
+ }

system.time(f(20))

user  system elapsed
   0.352   0.272  29.681

-------------------

I build my own R packaging under CentOS and do not have a 
packaging suitable for the architecture of that intermediate 
box -- installing R to a Debian testing box, and running the X 
forwarding connection only one hop, it is again visually 
rebdered MUCH slower.  I get:

system.time(f(20))

user  system elapsed
   0.644   0.212  45.089

yikes  ;)

I'll get a packaging built under CentOS 5 on that other 
architecture overnight, and supplement this post

-- Russ herrold