Massive speed drops at night

I have home broadband for about 2 years now. It’s been great, until a few months ago.

Every single day around 9pm my connections drops from 400mb to 20mb.

I restart my router and it picks up again. Is three doing something dodgy with our connections?

I’ve seen many posts about this, they claim congestion, but I don’t buy it.

Otherwise why is my speeds back once I restart my router?