Why does Bitcoin make my computer lag/freeze?
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.