I have exactly the same problem on a SunRay 1g, where the Xnewt X server is used. I'm using this on ubuntu 8.04.1 64-bit, desktop install. It's on a quite fast machine: dual quad core 3.2 GHz. After realizing that I had seen that problem elsewhere before, I realized that that was on my home PC: quad core 2.4 GHz, with nvidia 8800GTX 768 MB card (also ubuntu 8.04.1 64-bit), drivers for nvidia: 173.14. 

I've read on more forums that this problem it is proabaly related to nvidia for the hardware acceleration in their drivers or such. I don't think this is the case, as the SunRay 1g stuff uses an all-software Xnewt (which appears to be a derivative of Xorg 6.x), and the graphics card is certainly not an nvidia - not being used by that x server, except for a console (note that this is a server, not a workstation). Here's the xdpyinfo output, as the performance problem looks to be the same as for the nvidia people, they also reported this slashdot comment page panel dock, and they claim it to be one of the badly implemented extensions thus:

cowboytim@kwabber():~ > xdpyinfo -display :11
name of display:    :11.0
version number:    11.0
vendor string:    The X.Org Foundation
vendor release number:    60802000
X.Org version: 6.8.2
maximum request size:  16777212 bytes
motion buffer size:  256
bitmap unit, bit order, padding:    32, LSBFirst, 32
image byte order:    LSBFirst
number of supported pixmap formats:    2
supported pixmap formats:
    depth 1, bits_per_pixel 1, scanline_pad 32
    depth 24, bits_per_pixel 32, scanline_pad 32
keycode range:    minimum 8, maximum 254
focus:  window 0xa00011, revert to PointerRoot
number of extensions:    24
    BIG-REQUESTS
    DAMAGE
    DEC-XTRAP
    DOUBLE-BUFFER
    DPMS
    Extended-Visual-Information
    GLX
    LBX
    MIT-SCREEN-SAVER
    MIT-SHM
    MIT-SUNDRY-NONSTANDARD
    RECORD
    SECURITY
    SGI-GLX
    SHAPE
    SYNC
    TOG-CUP
    X-Resource
    XC-APPGROUP
    XC-MISC
    XFIXES
    XInputExtension
    XTEST
    XVideo
default screen number:    0
number of screens:    1

screen #0:
  dimensions:    1280x1024 pixels (361x289 millimeters)
  resolution:    90x90 dots per inch
  depths (1):    24
  root window id:    0x29
  depth of root window:    24 planes
  number of colormaps:    minimum 1, maximum 1
  default colormap:    0x21
  default number of colormap cells:    256
  preallocated pixels:    black 0, white 16777215
  options:    backing-store YES, save-unders YES
  largest cursor:    64x64
  current input event mask:    0x72003c
    ButtonPressMask          ButtonReleaseMask        EnterWindowMask          
    LeaveWindowMask          StructureNotifyMask      SubstructureRedirectMask 
    FocusChangeMask          PropertyChangeMask       
  number of visuals:    4
  default visual id:  0x23
  visual:
    visual id:    0x23
    class:    TrueColor
    depth:    24 planes
    available colormap entries:    256 per subfield
    red, green, blue masks:    0xff, 0xff00, 0xff0000
    significant bits in color specification:    8 bits
  visual:
    visual id:    0x24
    class:    TrueColor
    depth:    24 planes
    available colormap entries:    256 per subfield
    red, green, blue masks:    0xff, 0xff00, 0xff0000
    significant bits in color specification:    8 bits
  visual:
    visual id:    0x25
    class:    TrueColor
    depth:    24 planes
    available colormap entries:    256 per subfield
    red, green, blue masks:    0xff, 0xff00, 0xff0000
    significant bits in color specification:    8 bits
  visual:
    visual id:    0x26
    class:    TrueColor
    depth:    24 planes
    available colormap entries:    256 per subfield
    red, green, blue masks:    0xff, 0xff00, 0xff0000
    significant bits in color specification:    8 bits

I'v tried this with the ubuntu stock firefox, 3.0.1+build1+nobinonly-0ubuntu0.8.04.3 (which is 64-bit), and also with a downloaded firefox: firefox-3.0.1.tar.bz2 (32-bit).