It depends. If you have mostly N devices, and just a handful of infrequently used G devices, then the same will likely be fine. The N devices do have a bit more overhead when the network runs in mixed G+N mode, but it's usually minor. Slowdowns only happen when G devices are chatting too. B devices would degrade G networks just by being connected and idle.

http://www.smallnetbuilder.com/wireless/...ading-to-80211n has some tests showing the problem, and it can be a big hit at times. If you have a lot of G devices, a separate SSID may be best.

For me, I just isolate it by keeping my 5GHz N network at N only (no old A allowed), and the 2.4GHz band in mixed G+N mode.