Before you go any further I suggest you check that there are XP drivers for your laptop. If there isn't then theres nothing you can do and you will have to stay with Vista.
Theres various threads about dual booting and I'm sure you can have a look for them. I would suggest that XP won't be twice as fast as Vista, it will be faster but I'm thinking something like 20% not 100%.
Can you provide a link to where you read this as I did not know there was going to be an SP3 for XP.