I'll admit, my PC isn't the best. It's fairly old now, still running XP and fair to say a little clogged up.

Despite this it still performs fairly well in most cases, but Quest seems to run very slowly and sluggishly. Is this normal?

No, Quest runs fine on my PC. However I used to have an old (2005) laptop with XP and yes, Quest was a little slow sometimes. Upgrading to my new (used, from 2011) laptop made that go away.

Online or offline game? Offline it should be smooth - online you sometimes get pauses between commands and Quest's response.

Thanks.