There are only 3 properly separated Wi-Fi channels in thee 2.4GHz spectrum, and should have simply been called 1, 2 & 3
Channel 1 actually uses the frequencies listed for channel 2 and well into those listed for Channel 3
Channel 6 dips down uses the frequencies lusted for Channel4 4 through to 8
Channel 9 runs uses frequencies listed for Channel 9 through to 13 (available in some regions)
You take a much smaller performance hit having multiple APs using 1, 6 or 11 than it is to have APs using adjacent channels (say 1 & 2)
APs on adjacent channels give a 5 times greater hit than simply using the same channel.
The rule I have in my notebook (from my Ekahau course) is that 2 APs on the same channel will take a 55 meg connection down to about 5 meg (bad enough) but adjacent channels are likely to take the connection down to 1 meg
This is assuming a default channel width