It is, absolutely. And Wordpress is notoriously liberal with its queries and use of storage in relational database (unless something monumental has changed in the last year). There's a reason there are 8 million caching plugins.
If PHP is the slowest part of your application, you're probably in good shape. If your Wordpress application is slow, replacing PHP with Hack is like putting a souped up engine in a car with no gas in the tank.
PHP scaled poorly at Facebook not because it was PHP, but because it was Facebook. They would have run into the same (or worse) performance issues with most dynamically-typed languages.
If PHP is the slowest part of your application, you're probably in good shape. If your Wordpress application is slow, replacing PHP with Hack is like putting a souped up engine in a car with no gas in the tank.
PHP scaled poorly at Facebook not because it was PHP, but because it was Facebook. They would have run into the same (or worse) performance issues with most dynamically-typed languages.