Why does Bitcoin make my computer lag/freeze? Whenever I run bitcoin, my computer (Dell E4200 running Ubuntu 12.04, with AwesomeWM) lags quite heavily.

It will operate normally for 10-15 seconds, then 'freeze' for 10 to 15 seconds, during which time keyboard and mouse input seems not to register. Some actions are more affected than others. Chrome won't output keystokes in this textarea during such a lag, but I can change tabs freely. All the keystrokes show when the freeze ends.

I am running bitcoin-qt from this repository: https://launchpad.net/~bitcoin/+archive/bitcoin

I installed it with sudo apt-get install bitcoin-qt

Version shown during install:
Unpacking bitcoin-qt (from .../bitcoin-qt_0.6.2-precise0_amd64.deb) ...

I had the same issue when I was running Ubuntu Unity, and the 'original' bitcoin client.

My client is currently synchronizing with the network.

My total CPU usage is between 20-60%, with occasional spikes to 90+%.

Memory usage is steady at ~60%, and swap is ~5%.

Any clues to help me diagnose / solve this issue?

Thank you.
When you use the Bitcoin client for the first time, the client has to download the whole Bitcoin blockchain from the network. This can really (really!) take a while and because it is stored in your homefolder and therefore has to be encrypted, this take some CPU power as well. 

Apart from this, while your client runs, it helps verifying new transactions offered to the network, just like every connected Bitcoin client does, these calculations can take CPU power as well.

What CPU does your machine have?
You can try to use clients like MultiBit that do not download the whole blockchain, but only the part that is relevant to your coins.