More clear test cases:

    $tests = array(
        'empty' => array(),
        'x10'   => range(1, 10),
        'x100'  => range(1, 100),
        'x1000' => range(1, 1000),
    );

Result:

    empty       0.15 sec. total
    x10         0.65 sec. total
    x100        5.14 sec. total
    x1000      49.91 sec. total

Looks like array is copied while casting:

    # memory_limit=1G

    echo "Create array...";
    $array = array_fill(0, 10 * 1000 * 1000, null);
    echo " ok\nCasting...";
    $start = microtime(true);
    (bool) $array;
    printf(" %.3F sec.\n", microtime(true) - $start);

Output:

    Create array... ok
    Casting...Fatal error: Allowed memory size of 1073741824 bytes exhausted (tried to allocate 35 bytes) in ...

The workaround is to first count with arrays as the count is cached.

(bool) count($array);

Yes, "workaround" to pass thru the bugs maze. Welcome to PHP. Did you expect something else?..

Workaround to get the timings low in the test-script only. And I didn't meant that in a positive fashion. A workaround is always crap and expensive.

However, as internally it might be easy to get the array value count, the internal function that so far converts array into boolean could be optimized by looking for such array count instead of iterating through it.

And yes I did expect something else. I was quite astonished how expensive the bool cast is with arrays with more than 100 members. I didn't know that so far. I always expected casts to be quick, I hope this gets fixed.

Yes, the guess is correct - we copy the array, then "convert" the copy see  ZEND_CAST opcode handler http://lxr.php.net/xref/PHP_TRUNK/Zend/zend_operators.c#511
I don't see an obvious way to improve this without duplicating the conversion logic. You have an idea Dmitry?

Instead of (bool) count($array), you can use empty($array), which seems cleaner and more obvious.
Using empty() results in:
$empty      0.10 sec. total
$full       0.10 sec. total

Fixed in PHP 7.