Clean out old and unused settings in WPML's icl_sitepress_settings row in the wp_options table.
Depending on how many pages you have, if you use ACF, and if your pages are ordered differently in different languages, you may have a lot of data in the icl_sitepress_settings row in the wp_options table. On WPEngine any wp_options table row which is set to autoload and larger than 1mb will be deleted by a WPEngine cron job that runs overnight.
This is possibly just a partial solution as your config can continue to grow. I've seen in the site I was working in at the time that there were many old an unused values do to ACF structure changing and fields being removed.
This script will get the value of the wp_options row where the meta_key is icl_sitepress_settings. The custom_term_fields_translation and custom_fields_translation elements from the value of that row will be compared to the wp_postmeta table to see if the icl_sitepress_settings indices have a matching row in the wp_postmeta table where the meta_key is a match. If there is no match then the indice is removed from the icl_sitepress_settings. This will not remove any data from the wp_postmeta table.
Make a backup of your existing icl_sitepress_settings row in the wp_options table before running the script. And your whole db for that matter.
I have seen a fatal error happen when running the script while the WPML status bar was showing that it was processing items. I'm not sure if this was a coincidence or not but I would recommend running this script when you see that the status bar is not showing that it is processing items.