In PHP 7 (phpng), is_null is actually marginally faster than ===, although the performance difference between the two is far smaller.
PHP 5.5.9
is_null - float(2.2381200790405)
=== - float(1.0024659633636)
=== faster by ~100ns per call
PHP 7.0.0-dev (built: May 19 2015 10:16:06)
is_null - float(1.4121870994568)
=== - float(1.4577329158783)
is_null faster by ~5ns per call