I saw a number of negative reviews of past versions because of slowness, and post on forum [wordpress.org/support/topic/the-plugin-will-crash-bigger-sites/] documenting the problem.
( Note: I have to remove links from the review, as they are not allowed... Just add https: / / in front of links. )
It seems fixed in version 1.5 . Looking at code, class/RestResponse.php no longer does a big JOIN (old code: getMetafieldsWithPostTypes in [plugins.trac.wordpress.org/browser/integromat-connector/trunk/class/RestResponse.php?rev=2476182] ). Instead now (new code: [plugins.trac.wordpress.org/browser/integromat-connector/trunk/class/RestResponse.php?rev=2704186] ) it merely calls register_rest_field following the options you set for metadata to add to posts. From my shallow understanding of the problem, this makes the problem fixed, as it no longer queries database for a huge data.
I would recommend developers to address this better on the forum. The current negative score of the plugin is alarming. If the critical problem was actually solved now -> please communicate it clearly on forum, to hopefully get people to test (esp. the ones experiencing problem in the past), confirm, and make better reviews. And if the critical problem actually still exists, I'd like to know about it.
The forum thread [wordpress.org/support/topic/the-plugin-will-crash-bigger-sites/] is closed without any answer, which is a pity. The only acknowledgement from devs is short "Fix plugin slow down" on [wordpress.org/plugins/integromat-connector/#developers] at 1.5 -- this is too short 🙂