Optimize PHP MySQL fetch data from API and update table - php

Question

I have a table in MySQL with approx. 20 million rows.
id | word_eng | word_indic
I have to translate the english word (word_eng) into indian language (word_indic) using google translate api.
I have written PHP code which spawns multiple curl requests and fetches data from API and updates it into the table. But this process is quite slow, about 100 to 200 words per second.
I am using RollingCurl for multi curl.
Whats the best way to make it as fast as possible?
Below is my code. I am running this as a cron job.
<?php
include_once('db.php');
include_once('functions.php');
include_once('rolling-curl-master/RollingCurl.php');
$table = $argv[1];
$q = "SELECT * from $table where word_indic is null limit 500000";
$result = $conn->query($q); $n = 0;
$urls = array();
while ($row = $result->fetch_assoc())
{
$id = $row['id'];
$word = rawurlencode(getName($row['name_eng']));
//getName is a simple function which does some trimming and cleaning up of string
$url = 'https://www.google.com/inputtools/request?text='.rawurlencode($word).'&ime=transliteration_en_te&id='.rawurlencode($id);
array_push($urls, $url);
}
//print_r($urls);
unset($url);
$rc = new RollingCurl("request_callback");
// the window size determines how many simultaneous requests to allow.
$rc->window_size = 300;
foreach ($urls as $url)
{
// add each request to the RollingCurl object
$request = new RollingCurlRequest($url);
$rc->add($request);
}
$rc->execute();
function request_callback($response, $info)
{
// parse the page title out of the returned HTML
if (preg_match("~<title>(.*?)</title>~i", $response, $out)) {
$title = $out[1];
}
//echo "<b>$title</b><br />";
//print_r($info);
$parts = parse_url($info['url']);
parse_str($parts['query'], $query);
$id = $query['id'];
$text = $query['text'];
//echo "<hr>";
$trans = json_decode($response)[1][0][1][0];
global $conn; global $table; global $urls; global $n;
if ($trans != '' and !preg_match('/[a-z]/', $trans))
{
$conn->query("update $table set word_indic='$trans' where id='$id'"); $n++;
}
}
?>