i think you'll only see overhead on the device you are walking. that is, the device's CPU and it's interface handling the traffic for the walk. this has been my experience. there might be some older devices which could potentially crash with a heavy mib walk. i think Cisco has fixed most of them to prevent this type of intense mib query.
your overall network traffic/performance will probably not show any impact, unless of course, you're walking numerous devices at the same instance, then that might be considered DDos and could impact many users.