importprivkey is slow because it does a rescan of the entire blockchain, and blocks most other operations while it's running. It's not intended to be used frequently in its current form.
Thanks Pieter - I hadn't considered this as it was a newly generated vanity gen private key.
I guess waiting for approx. 10 minutes was not patient enough (only using a laptop).
Luckily it doesn't look like I damaged anything by shutting down bitcoin-qt (and also the import seemed to work fine).
Perhaps a flag to not scan could be considered (when you know you are dealing with a brand new private key like I was)