WordPress HTTP Error 500 and Slow Loading Sites

If your WordPress /wp-admin is slow to load sections or throws an HTTP 500 error when trying to update a theme or page, then you may have a memory limit issue.

Check your wp-config.php file for a WP_MEMORY_LIMIT defines. If the value is too low — then increase it to 128M or more.

define( ‘WP_MEMORY_LIMIT’, ‘128M’ );

If that doesn’t work, you may also try increasing the memory from the PHP.ini file:

memory_limit = 96M;

Lastly, you can try editing the .htaccess file by adding this value:

php_value memory_limit 96M

For more info see: https://wpe.approveme.com/article/101-increase-memory-limit