This is the issue with WiFi ...when I did my selfbuild .. many people laughed as I was putting in CAT 5 to every room (multiple in fact) saying that I was wasting my time and WiFi is the way to go.
It's not ... I have WiFi ... but go 2 rooms away and performance is down by 50%. You can help yourself ... get hold of a free app like Insider3 (insider3.software.informer.com/?)
It will show you what channels are busy ... then you can set your WiFi Router to work either on a free channel or avoid busy channels.
Routers are getting better with Chn hopping, and dual frequency but still not as good as looking for your self.
Unfortunately WiFi (802.11 standard)uses 2.4GHz spectrum for b & g devices ... the most common type. This is same as allocated to Amateur Radio, Microwave ovens, Movement detectors,(incl in-car alarms) baby monitors, cordless phones and Bluetooth.
So any of those can make a chn busy