That's an interesting read! I performed some tests.

Executing this took on average 3.31 seconds.

<?php

namespace Foo\Bar;

// one hundred million times
for ($i = 0; $i < 100000000; $i++) {
$a = strtoupper('Hello World');
}

?>

While executing this took on average 3.195 seconds.

<?php

namespace Foo\Bar;

// one hundred million times
for ($i = 0; $i < 100000000; $i++) {
    $a = \strtoupper('Hello World');
}

?>

That's a 3.47% performance boost.

Here you might think: 3.47% is not a lot. But...

I ran these two other pieces of code:

<?php

namespace Foo\Bar;

// one hundred million times
for ($i = 0; $i < 100000000; $i++) {
    $a = 'Hello World';
    strlen($a);
}

?>

<?php

namespace Foo\Bar;

// one hundred million times
for ($i = 0; $i < 100000000; $i++) {
    $a = 'Hello World';
    \strlen($a);
}

?>

And in this case, it's much faster! The first piece of code took on average 2.08s to execute, while the second took on average 1.53s to execute.

That's a 26.44% performance boost.