Daily slowdown in our home wifi (all usage points)

Every day around 5 or 6 pm, our home wifi slows down to about 30% of normal.  We have one PC and one home network connection hard-wired to the gateway, plus multiple TVs, game systems, and phones connected through wifi.  Every device sees the degradation in performance.  Sometimes some of them drop the connection completely.  I want to emphasize, this happens every day.  Re-starting the gateway sometimes helps, sometimes not.  It is not reliable or repeatable.