Hmm... if you mean system speed then not really even though onboard sound does use the CPU, which is why when I have used one and done a complex task, the sound cuts out or jumps which isn't good at all. Plus you get better sound quality with one that plugs into a PCI port.
With games, the speed would b noticable I would imagine if alot of sound channels are being used.
If you like music though for example, I would really consider switching.
I hope this helps