Oneiric crawls on my 1 GB RAM laptop

I upgraded to Oneiric on my Lenovo N100, 3000 series laptop.

Oneiric literally crawls on this laptop and I'm unable to understand why.

 I see my RAM usage, and it's usually never 500 MB (unless I have some app on or leave Opera browser with its 200 MB appetite on for long).

Despite this, my apps don't open up quickly, the shutdown takes forever, and the "alt+tab" is so slow I don't even use it anymore -- I just open windows in different virtual desktops and do ctrl+alt-arrow to navigate between different windows.

Maverick worked like a charm on this computer (I went straight from Maverick to Oneiric), and every other version I've had before that (Lucid, Karmic, Jaunty, Intrepid, and Hardy) have all worked amazingly fast. In fact, it would shut down within 5-7 seconds. But now, it takes longer for that shutdown screen to even show up, and then that screen goes through 3 - 4 rounds of those dots before the system completely shuts down.

How do I figure out what's wrong? I went to a Best Buy and got the hardware checked out. They said everything seemed OK. Here's another weird thing: I created a startup disk on a 16 GB external portable hard drive with Kubuntu 11.10 on it. When I use Kubuntu from that USB disk on this same laptop, it works faster than Oneiric does from the hard disk. Is that normal?

Let me know if I can use the terminal to understand if there's something wrong, or if I should just put a lighter version of Ubuntu and forget about Unity before I get a new, more powerful laptop.