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2.2 Data tab shows your post types and custom fields. To do so, WPSolr has to scan the wp_postmeta table, which takes a long time and memory resources on the server for 1.4 million post types.

I ran out of memory for the browser tab

This should not happen, unless you have many distinct custom field names in your wp_postmeta. Usually, you should “only” have tens of custom field names for millions of custom field values.

Try to find out how many custom field names and how many custom fields you get in your database.

Can you try this SQL statements?

SELECT  distinct meta_key
 FROM    posts as POSTS INNER JOIN postmeta as POSTMETAS
 ON      POSTS.ID = POSTMETAS.post_id
 WHERE   POSTS.post_type = 'your_custom_post_here'

If true, this means you have 1 million+ custom field names in your system (not values, names). Screen 2.2 tries to display those 1M+ names, which is impossible.

You can edit wpsolr-free/wpsolr/core/classes/models/post/class-wpsolr-model-meta-type-post.php, function get_fields() {...}

Or comment the use of this function in file wpsolr-free/wpsolr/core/dashboard/dashboard_settings.inc.php:
$model_type_fields = $model_type_object->get_fields();

Your modifications will be lost in next update, but it should be enough as a workaround.