Having this same issue on a shared-host provider, getting them to bump the PHP memory limit fixed the problem. I believe the issue stems from the use of wp_dropdown_users in wordpress-seo/admin/pages/metas.php. Our subscriber list is over 20k users and rendering that dropdown is pretty inefficient. For the time being I altered line 93 of the metas.php file:
diff --git a/wp-content/plugins/wordpress-seo/admin/pages/metas.php b/wp-content/plugins/wordpress-seo/admin/pages/metas.php
index 95782dd..f4e2605 100644
--- a/wp-content/plugins/wordpress-seo/admin/pages/metas.php
+++ b/wp-content/plugins/wordpress-seo/admin/pages/metas.php
@@ -90,7 +90,7 @@ if ( ( isset( $_GET[ 'updated' ] ) && $_GET[ 'updated' ] == 'true' ) || ( isset(
echo '<h2>' . __( 'Author metadata', 'wordpress-seo' ) . '</h2>';
echo '<label class="select" for="">' . __( 'Author highlighting', 'wordpress-seo' ) . ':</label>';
- wp_dropdown_users( array( 'show_option_none' => "Don't show", 'name' => 'wpseo_titles[plus-author]', 'class' => 'select', 'selected' => isset( $options[ 'plus-author' ] ) ? $options[ 'plus-author' ] : '' ) );
+ wp_dropdown_users( array( 'who' => 'authors', 'show_option_none' => "Don't show", 'name' => 'wpseo_titles[plus-author]', 'class' => 'select', 'selected' => isset( $options[ 'plus-author' ] ) ? $options[ 'plus-author' ] : '' ) );
echo '<p class="desc label">' . __( 'Choose the user that should be used for the <code>rel="author"</code> on the blog homepage. Make sure the user has filled out his/her Google+ profile link on their profile page.', 'wordpress-seo' ) . '</p>';
echo $wpseo_admin_pages->textinput( 'plus-publisher', __( 'Google Publisher Page', 'wordpress-seo' ) );
echo '<p class="desc label">' . __( 'If you have a Google+ page for your business, add that URL here and link it on your Google+ page\'s about page.', 'wordpress-seo' ) . '</p>';
Basically just adding the 'who' => 'authors' option to the wp_dropdown_users args.
