performance issue when using request_multiple

The following code will cause serious performance problem:

```php
do {
    $status = curl_multi_exec($multihandle, $active);
}
while ($status === CURLM_CALL_MULTI_PERFORM);
```

it will be much better if changed as follow

```php
do {
    $status = curl_multi_exec($multihandle, $active);
    usleep(1000);
}
while ($status === CURLM_CALL_MULTI_PERFORM);
```