GPU slower than CPU?

[Konvict]

I recently 'installed' a GPU I was given by my dad, its an 'nVidia GeForce GTS 240'
I spent the day setting it up, then to realize that a dictionary attack with oclhashcat-plus was 10 minutes, while a dictionary attack with original hashcat took apporx: 5 minutes.
Why could this be, is it just plain and simple that my GPU specs aren't good enough or could there be something im missing here?

Thanks for your help, I can give you more information if needed.