wp_usermeta query hanging for a few seconds before loading - mysql

Question

I have a query that I wrote a couple of years ago for a membership website. I'm not extremely well versed in using $wpdb (or MYSQL code in general) to write custom queries, and the site has grown quite a bit. There are about 150k rows in the wp_usermeta table now, and the page where the query runs hangs for a couple of seconds now before loading. I expect that this will get worse as time goes on and the site gains more users.
Any help in figuring out how to speed this query up would be greatly appreciated.
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$limit = 15;
$offset = ($paged - 1) * $limit;
$key = 'first_name';
$sql = "SELECT SQL_CALC_FOUND_ROWS {$wpdb->users}.* FROM {$wpdb->users}
INNER JOIN {$wpdb->usermeta} wp_usermeta ON ({$wpdb->users}.ID = wp_usermeta.user_id)
WHERE 1=1
AND wp_usermeta.meta_key = '$key'
AND wp_usermeta.meta_value <> ''
AND wp_users.user_registered < '2014-01-30'
ORDER BY wp_usermeta.meta_value ASC
LIMIT $offset, $limit";
$members = $wpdb->get_results($sql);
$found_rows = $wpdb->get_var("SELECT FOUND_ROWS();");
foreach ($members as $member) { // display member info here }
*Note: I am paginating...not displaying all results on one page.

Answer 1

A couple of things here.
First, you seem to be ordering by user first name, but not retrieving or displaying it. That's a bit odd. You could order by wp_users.user_nicename, and omit the join to wp_usermeta. That will save some time in joining. That column's value is set by the user in her profile, and reflects the identity by which she wants to be known. So, it's an appropriate ordering column, most likely.
Second, you're retrieving all the columns (*) in wp_users. Can you avoid retrieving them all, and rather enumerate the one you actually need? That will save time in sorting.
Your query would become this.
$sql = "SELECT SQL_CALC_FOUND_ROWS
id, user_login, user_nicename, etc etc
FROM {$wpdb->users}
WHERE user_registered < '2014-01-30'
ORDER BY user_nicename ASC
LIMIT $offset, $limit";
You are stuck, as you are paginating a large number of users, with some inescapable inefficiency. But this should be better than what you have.