se non usi le curl ma ti scrivi il downloader da solo si mentre per quanto riguarda le curl "forse"

se non erro le curl hanno un meccanismo per notificare una funzione ogni qual volta scaricano un pacchetto di dati quindi se li ci metti uno sleep potrebbe (con molti punti interrogativi) funzionare

value should be a string that is the name of a valid callback function for the following values of the option parameter:

Option Set value to Notes
CURLOPT_HEADERFUNCTION The name of a callback function where the callback function takes two parameters. The first is the cURL resource, the second is a string with the header data to be written. The header data must be written when using this callback function. Return the number of bytes written.
CURLOPT_PASSWDFUNCTION The name of a callback function where the callback function takes three parameters. The first is the cURL resource, the second is a string containing a password prompt, and the third is the maximum password length. Return the string containing the password.
CURLOPT_PROGRESSFUNCTION The name of a callback function where the callback function takes three parameters. The first is the cURL resource, the second is a file-descriptor resource, and the third is length. Return the string containing the data.
CURLOPT_READFUNCTION The name of a callback function where the callback function takes two parameters. The first is the cURL resource, and the second is a string with the data to be read. The data must be read by using this callback function. Return the number of bytes read. Return 0 to signal EOF.
CURLOPT_WRITEFUNCTION The name of a callback function where the callback function takes two parameters. The first is the cURL resource, and the second is a string with the data to be written. The data must be written by using this callback function. Must return the exact number of bytes written or this will fail.
Potresti provare ad usare o la CURLOPT_READFUNCTION o la CURLOPT_WRITEFUNCTION, in ogni caso guarda nei commenti per vedere se qualcuno le ha utilizzate o guarda su google