In last two months, WPML team have rewritten plugin String Translation completely, and added many performance improvements to Multilingual CMS and Translation Management plugins.
Technically, it was decided to get rid of storing strings in database.
A typical admin page which opens a page builder, WooCommerce and several other big plugins could load over 30K strings from the database. Instead, now WPML generates own .mo files with translations once, and updates them when needed. Details can be found in company blog.
It is hard to name first feedbacks from clients other than rapturous. In the particular case, it was confirmed average decrease in page loading time by 64%, and average decrease in database requests by 37%.
Developers continue to work on performance. On Friday, it was published the new beta6 version plugins with significant improvement of performance on sites using ACF.
Beta-version is available for testing to WPML users. Stable version is expected to be released in the beginning of Autumn.