I am having an issue with both Unity and GNOME Shell in Ubuntu 10.10.
It is performance related. I should not have any issues. I have a new
computer with a quad core and loads of RAM and 1 GB dedicated video
card. My usual desktop environment is KDE 4.6. On that I have nine
activities with four virtual desktops in each (that's 36 work
environments) and it runs fine with lots of plasmas and widgets and
applications open. However, in both GNOME shell and Unity it is
sluggish from the start.

I have GNOME Shell installed, but you cannot access it from the
session menu at login because I have full GNOME installed. The way I
have to launch it is with the gnome-shell --replace command after
being in GNOME. I have heard that GNOME shell fights with GNOME panel
and other parts of GNOME and they do not get along well. When it first
launches it works fine, but soon as I open a single application the
mouse lag is ridiculous. It is jerky and I can't get much done. My
problem might be related to Compiz which is the window manager fro my
regular GNOME. I need to remember to shut it off before launching
Shell. I think that it turns off by itself, but am not sure. If I try
to use Compiz after GNOME Shell is launched then it locks up the
computer. I can't even REISUB. So I figure that it must be changing to
another window manager, but whatever it is, it isn't working for me.

I am wondering if I remove GNOME and only go with GNOME shell if I get
an entry in the Session manager and if this will clear up my problem.
I recently logged into Unity and it is similarly slow. I had not been
previously concerned because Unity has always sucked performance-wise
due to Mutter and GNOME Shell is still alpha. But the GNOME folks say
that it is ready and they are only tweaking. GNOME Shell uses Mutter
and that confuses me because Canonical is dumping Mutter in Natty
because it under-performs.

I have got used to using Activities in KDE as opposed to using virtual
desktops and would like to try the same in GNOME with GNOME Shell to
make a fair comparison. Is anybody running GNOME Shell alone without
regular GNOME? Can I launch it at login? If you are not having
performance issues tell me what you are doing differently.