improve performance

[drzraf]

This importer is great!
But it's still slow, importing a thousand of nodes having some dozens ACF is still a task taking tens of minutes.
One issue is the time spent into making thousand of individual INSERT INTO wptests_postmeta.
Thus I wonder if this couldn't be bulkified.

• This suggests wp_defer_*_counting(true) which Importer already does.
• It also suggest disabling autocommit (which it does not).
• There is also the possible remove_action('do_pings')
• ability to disable fetch_attachments from the command-line PR extend the options that wp-cli can pass to the import #145
• reduce WXR verbosity/size (improve XML readability) PR Parsing <wp:postmeta> using an attribute-based/less-verbose XML structure #148
• fetch attachment asynchronously or in a subsequent import
  Don't you think having a some (WP-core or not) bulk_add_post_meta() could dramatically improve import time?