How not to let the speed drop?

Question

Computer networks

Good day.
At home, the Internet is distributed via wi-fi to laptops. The computer is connected by cable. The problem appears when online video is turned on on any of the computers. The Internet on other devices becomes terrible: ping jumps, speed drops, pages do not load. The Internet speed is 15Mbps, when the video is watching it drops to 3. What could be the problem? And how can it be solved?
ps wi-fi TPLink WR-720N
UPD1
Is it possible to manually set the service priority?

Answer 1

What can be said?
A well-encoded video has less than 1 bit per pixel. On YouTube, somewhere around 0.1 ... 0.2 bits / pixel, in well-encoded videos from Rutracker 0.3 ... 0.4. If we take 640 × 480 and the reference 0.2 bits / pixel, we get 1.5 megabits, if 720p is already 4 megabits. So if you're not watching 1080p, there should be a decent chunk left. And the router, even the cheapest one, should cope, not 2007 in the yard. So far I see one of the two.
1. Features of provider QoS. Then only the transition to a more expensive Internet.
2. A cheap router provides QoS too straightforwardly for different users.