Why does my wireless slow down at time?

I am a host parent and I host an international student from China. So my problem is when he came and stayed with me, my internet always slow down and sometimes we both could not even get onto the internet. I have the best wireless router and very high speed internet. I called Verizon several times and they said my internet has no problem at all. And It indeed works perfectly fine when my student was going back to China in summer. In addition, my HP printer and Sonos also connect to the wireless, I tried to print something last night and tried to listen to music this morning, and they both cannot be connected to my Ipad or iphone. So I reset the router and all the sudden the Sonos is connected, the printer started printing, the internet goes to very high speed. I don't know what has happened to the wireless, but there must be something that slows down the internet. I am sure my student would go on to some Chinese websites, but is that a problem that impacts the internet? Please help! Or please tell me who I could call to get my problem fix. Thank you very much!

Read more: Why does my wireless slow down at time?