Micro optimization isn't worth it.
You had to do it ten million times to notice a difference, a little more than 2 seconds
$a===NULL; Took: 1.2424390316s
 is_null($a); Took: 3.70693397522s
difference = 2.46449494362
difference/10,000,000 = 0.000000246449494362
The execution time difference between ===NULL and is_null is less than 250 nanoseconds. Go optimize something that matters.