Since upgrading to the latest version the indexer is considerably slower and began throwing Allowed memory size exhausted errors. Is there any documentation yet about what changed specifically in the indexer from 1.7 to 2.2.3 that might help narrow down what’s going on?
I would recommend using WP-CLI for indexing large DBs.
In 2.0.0 introduced some major changes in code base. Plugin no longer uses wp-async-task and task queue system. Not sure, if these changes is directly related to memory issues, but it worth exploring.
So from 1.7 to 2.x we actually bumped the default number of posts to index per batch from 50 to 100.
Performance-wise, it should be better than before because we removed a lot of complexity.
To reduce the number of items to index per batch back to 50, you can use the following: https://community.algolia.com/wordpress/frequently-asked-questions.html#the-indexing-is-slow-can-i-optimize-the-required-time-
Let us know if that solves your issue.