Why do youtube videos freeze?

Question

YouTube

I bought a xiaomi prefix, launched it and then noticed that the rollers were slowing down. The picture freezes, the sound goes on, then the sound freezes. Video in 480p. I didn’t see other users complaining about such problems, so I think it’s a sin on the router (it’s old, decrepit, it’s in another room, and even a tp-link), but the connection is 3-4 sticks.
Internet 50 Mb / s, when connected via a PC wire, everything loads quickly. The rollers slow down when there is no load on the network at all.
Player: XIAOMI Mi TV Box S EU