While optimizing a function in PHP, I changed

if(is_array($obj)) foreach($obj as $key=>$value { [snip] } 
else if(is_object($obj)) foreach($obj as $key=>$value { [snip] } 

to

if($obj == (array) $obj) foreach($obj as $key=>$value { [snip] } 
else if($obj == (obj) $obj) foreach($obj as $key=>$value { [snip] } 

After learning about ===, I changed that to

if($obj === (array) $obj) foreach($obj as $key=>$value { [snip] } 
else if($obj === (obj) $obj) foreach($obj as $key=>$value { [snip] } 

Changing each test from is_* to casting resulted in a major speedup (>30%).

I understand that === is faster than == as no coercion has to be done, but why is casting the variable so much faster than calling any of the is_*-functions?