I think it might be a matter of coincidence that things are better with Guest Wi-Fi turned on or off - do you have some empirical evidence?
Normally on a domestic Wi-Fi access points it is not possible to use separate channels for different base stations on the same band (i.e. 2.4GHz or 5GHz). You may have the have an SSID bound to one band (i.e. 2.4GHz) and another SSID bound another (i.e. 5GHz). Much of the 2.4GHz range overlaps anyway so unless you were able to use distinct channels actually having the SSIDs on the same frequency would be better than being separate.
The first thing to do is check you have the latest firmware - often new firmware is release to address Wi-Fi performance issues.
Wi-Fi analyzer should show you all the other access points in the area and what channel they are using. I would recommend you choose from either 1, 6 or 13 (in the 2.4GHz band) - sometimes the automatic option on a Wi-Fi router does not perform well.
To asses which band to use as a first stab (and this does not take into account signal strength or use) just total up the number of access points using channels 1, 6 and 11. If you have access points using say 2 or 8 then you will have to count them twice for the channels they overlap - i.e. 2 overlaps the signals for both 1 and 6.
If you have a draw or close draw then choose the channel that has the least stronger signals in there.