The main reason behind websites being slow is overloading of hosts.
The benefit of using PHP over other compiled languages is the ease of maintenance. PHP has been designed ground up to efficiently handle HTTP traffic, there is less to build in comparison to building using other compiled languages. In addition to this, merging changes is an easier task since the server doesn’t need to be recompiled and restarted (which needs to be done in case of using a compiled binary- FastCGI).
PHP, when properly written, can be scaled to a great extent. Other limiting factors include the database engine that is used.