bug: jit.window with ;max hidemenubar front

hey cycling team,

if do this:
- hide the menubar with ;max hidemenubar
- show the jit.window fullscreen (doent matter isf you use the fullscreen 1 or the rect message)
- send the front message to the jit.window

then framerate goes down to 1-3 fps

my system is:
actual max5 version (5.1.4)
mac mini 3,1
mac osx 10.6.3

there are 2 workarounds i found:
1) do not send the front message to jit.window
move the patcher offscreen
with a window size -200 0 -10 100, window exec to the thispatcherobject

2) sending a ;jitter glreadback fbo message to the mainpatcher

my question is:
does a a ;jitter glreadback fbo message slow down my performance ?
i could not find a documentation.

gisela