From the main Gnome menu choose, "System" -> "Startup Applications".
Now you should be greeted with a window similar to the below one which I got in Ubuntu 10.10.
From this menu, you can remove the services that you don't like.
*. Bluetooth Manager -
If you don't Bluetooth service, then go ahead and disable it (click on it).
*. Certificate and Key storage -
This service is being used by Gnome desktop to store sensitive date such as user names and passwords, etc securely by encrypting them. So, don't change anything here, leave it be (although Gnome did replace it with seahorse from the version 2.2 if I remember correctly, but still keep it).
*. Check for new hardware drivers -
Well, I think you can guess what this is. Unless you really want the OS to automatically check for new drivers (should disable it if you don't have any Internet connection/s), then it is safe to disable it (but remember, make sure do a manual check for your drivers from time to time... who knows... there might be some major bug fixes. In doubt, then leave it enabled which is the default setting).
*. Disk notifications -
When your disks get full or near... then this is the service behind those "notifications". So if you don't want them, then disable it too.
*. Evolution Alarm Notify -
If you don't use Evolution or have no idea what the heck that is.. then disable it too.
*. Gnome Login sound -
Well if you don't fancy any greeting drums or whatever the ones Ubuntu uses ;-) when logging in, the disable it too.
*. GSettings Data Conversions -
Leave it alone (my fingers hurt... it kinda long story, but in short, Gconf is a bit similar to the Registry in Windows, in which Gnome keeps its desktop configuration files, etc).
*. Power Manager -
Leave it (especially if you have a lap top and want to set settings such as automatic screen off, etc).
*. Print Queue Applet -
If you don't have or don't use a printer, then disable it too.
*. Pulse audio sound system -
Leave it (I kinda sound like an arrogant ordering dude.. don't I ... sorry about that .. I'm usually pretty cool).
*. PulseAudio Sound System KDE Routing Policy -
Please leave it (ahhh better).
*. Remote Desktop -
Well, unless you want to login to a remote computer then you should disable it (most of the users actually don't need it).
*. Secret Storage Service -
Another one to leave alone.
*. SSH Key Agent -
Don't touch it either.
*. Ubuntu One -
Well, even though, by default every registered Ubuntu users get an on-line Ubuntu storage for free, still most won't be using it. So if you don't need this, then disable it too.
*. Update Notifier -
To be honest I'm not sure what updates this means... I guess it is there for the Gnome desktop rather than Ubuntu ... but as they say "when in doubt, go around" ... was it?. Anyway if you don't care much about updates or have no Internet connection at all, then disable it otherwise, leave it.
*. User Folder Updates -
Must be enabled.
*. Visual Assistance -
If you don't have any especial requirements due to disabilities (on screen keyboard, narrator, etc) then disable it.
And as last note -
I like the default Gnome theme called the "Clearlooks", and this may be due to the fact that I have an old laptop, but still in comparison with the Ubuntu themes, the ClearLooks feels much light to hand and fast as well.You can do it by right clicking on your Gnome desktop and choose ...
"Change desktop background" -> Click on the "themes" tab and choose "Clearlooks" from that window.
Lastly - Again access the above window, this time click on the tab called "Visual Effects" and choose "None" (yikes ;-) ) for disabling any GPU/CPU hungry effects which should help to speed things up a bit.
So, if you want your Ubuntu desktop to be fast and wanna log-in to your Gnome desktop with some speeds... then those should help. If you have any issues... then throw some me comments.