Previously `’include_children’ => false` was added to almost every taxonomy query on WordPress.com. When a term query is processed, WordPress first looks to see if the term is shared. When global terms were enabled on WordPress.com (before the 4.4 merge) this would always return true.
If the term is shared it will not include the children term. If the term is not shared, it will include the children.
As of 4.4 Terms have all been split on WordPress.com which means that this now adds `’include_children’ => true` to almost all taxonomy queries. This can have a very significant performance impact on code that was performant previously. In some cases this change means that queries that used to return almost instantly will time out. We therefore now recommend `’include_children’ => false` to be added to all taxonomy queries when possible.
In many instances where the wanted behaviour is to grab all posts that are either the parent or a child term this can be replaced by only querying for the parent term and enforcing inside a save_post() hook that if a child term is added that it’s parent term is also added. A one time WP CLI command might be needed to ensure previous data integrity.