I have a Lenovo G580 (4GB DDR3, ~500 GB HDD, Intel i3). I installed Ubuntu using the in-Windows installer on my Windows 7. Ubuntu was (and still is) slightly sluggish.
By sluggish I mean, the OS GUI is perfectly responsive, but an App's GUI sometimes just hangs/freezes for a second, while the rest works, but the app itself can't be accessed (happens with Google Chrome mostly, but other applications suffer, too) and it fixes itself after a few seconds.
I recently installed Windows 8 and it appears to be the same.
Another problem I am experiencing is that, when I try to boot it up, from the Windows 8 boot menu, it doesn't work right away. I have to select Ubuntu, and then I need to reboot the laptop (long press the power button on this laptop), which then automatically skips the Windows 8 boot menu and goes to the Ubuntu login screen. Also, it takes a really long while while it loads up and gets to the login screen (at first it's just staring purple/pink for around 20 seconds, and then when the logo and that load indicator appear, it is loading for 2 to 5 minutes).
I really love using linux for development (since I don't have to constantly use git to test it). But, this sluggishness is really getting in the way. :(
sudo apt-get install gnome-panel
log out click the circular icon next to your user name and select "Gnome Classic" or "Gnome Classic (No Effects)". You can always change back to Unity later if it does not solve your problem