Similar threads on new post creation

[squishi]

I noticed that the similar threads suggestion box is put below a newly created thread on here.
On my forum, I've never noticed this behavior.

Is this a modification? Or can I set it in the vbulletin settings?
How much processing power does that feature need?

[MysCha]

admincp > vboption > Message Searching Options > Automatic Similar Thread Search > Yes

Setting this option will cause a search for similar threads to be automatically done when a new thread is posted. These similar threads are then linked to from the newly posted thread's page. This can have an effect on performance.