I think you should be able to run virtually any distro, IMHO the bottlenecks on a laptop is far greater than just the USB 2.0 drive enclosure.
The fastest Linux is the one you build yourself, you have control over what libs and extras gets installed.
Example, I tried Xubuntu in the past (last year) on my netbook and it was a frustrating experience, SLUGGISH!
Last week, after Cinnarch went over to Gnome, I installed pure Arch Linux on the netbook with XFCE and it flies! I cannot believe how smooth everything is, and composit with a couple desktop effects is enabled!
For that reason I would suggest something like Arch Linux, Debian, Slackware, and their kin. Don't throw away
Bodi Linux! I have it installed on a P3 laptop and it works quite well.
You can Google for "lightweight linux distro" and pick the one that looks promising. Maybe Mint with mate desktop? Ubuntu, Fedora, OpenSUSE, etc. I THINK will be a bit much.
But that is the beauty of Linux, try Ubuntu, if you don't like it, try something else.