simplest answer of your question

    when PHP becomes slow?

only when you use it inefficiently, Although some languages are slower in their implementations but PHP is not one of them. The main reasons of PHP slowness is the reason that most PHP programmers( also most of other languages programmers as well) care very less about optimizations. They include huge CSS and JQuery libraries that they never used in their code. PHP is very friendly with HTML unless ASP.net so some programmers use it carelessly. Biggest social network facebook uses PHP very well.