Concurrent HTTP request loop
I don't see what could be causing ever-decreasing performance since the only state outside the main loop is a counter, but adding calls to memory_get_usage as @jsanc623 recommends would help with that.

But you can definitely improve the performance by avoiding the loop over $results for every URL returned. Take advantage of PHP's array which is a hash table providing O(1) lookup.

First, make the URL the key for the results array.

foreach ($items as $item) {
    if ($item-&gt;img !== '') {
        $url = $item-&gt;img;
        $results[$url] = array(
            'id' =&gt; $item-&gt;id, 
            'url' =&gt; $url,
            'size' =&gt; null
        );
        $rollingCurl-&gt;get($url);
    }
}


Next, use isset instead of array_search and looping.

// foreach ($results as $key =&gt; $value) { ... } becomes

$url = $request-&gt;getURL();
if (isset($results[$url]) {
    $results[$url]['size'] = $length;
    File::append($filePath, $results[$url]['id'] . ',' . $length . "\r\n");
}


Since you're placing each URL into the array before making the GET request, the if isn't even necessary.

Separately, you can delay writing to the file until the end of the inner loop so you write one big chunk rather than one thousand tiny chunks. It may be that I/O is more costly, and perhaps you're getting killed by seeking to the end of an ever-growing file? Seems unlikely but maybe after a million writes it adds up.

General Review

There are a few things that could be cleaned up here as well.


Consistent formatting: put your opening brace on the same line or the next, but do it consistently.
Blank lines before the closing brace seem misplaced to me. Use blank lines to logically group related code, though functions are preferred to whitespace.
Simplify testing if the image URL exists

if ($item-&gt;img !== '')


can be simplified to

if ($item-&gt;img)


to skip null values as well. Even if the field cannot be null, it's still easier to read.