Netflow is done in hardware on some platforms, so any CPU increase will be minimal. On other platforms it's done in software - and as with any software feature, there will of course be some CPU impact.
Probably the biggest CPU impact is when the netflow cache is too small for the amount of flows, and netflow enters "emergency expiry" to quickly export some flows to make more space in the cache.
If it's doing that a lot then you should check your traffic profile and re-size the cache accordingly. You can also adjust the flow lifetimes.
Commands are:
ip flow-cache entries
ip flow-cache timeout active
ip flow-cache timeout inactive
