On N270 Atoms, whole-disk AES encryption works perfectly fine, and the only time I notice a slow-down is when I'm running a benchmark program side-by-side with a model that has an unencrypted drive. For regular browsing and e-mail (which is what the person asking the question listed as a qualification), it's a non-issue.
As some others have posted, and what my local police have told me, the laptop will likely have been sold for cash in less than 24 hours. Unless you are being targeted specifically for something of significant value such as corporate IP, it's unlikely that anyone is going to spend the time to try to unencrypt your drive.
But other threats still loom...
If you plan on connecting to any network, you will expose your machine to any network-based threat, so you ought to harden your machine accordingly.
Make sure you still have a strong password for your account login. If your machine is in hibernate, the crypto authentication prompt will stop them, but if your machine was sleeping, it'll return to the OS prompt.
The one scenario where you're not protected at all is if the machine is powered on, logged in, and someone grabs it by force. I realize there are proximity-based USB dongles that will lock the screen when the remote adapter is beyond range, but this may be far too impractical to use. A USB security dongle sticking out the side is a quick recipe for a broken USB port...