The problem most likely 2, which reduces to one.
1. Processor poor router just can't cope with the amount of data chokes. Or the RAM is not enough. Then, as they say, no comments — so everything is clear.
2. Have WiFI is not available, MIMO — Multiple In Multiple Out. It is very much affected if you have very many devices connected to one access point. Read about it in this article: habrahabr.ru/post/132247/
As you can see, it all comes down to the fact that you have too many devices on one router. So what people suggest is true, you will have to increase their number.