The main things that affect indexing time in Magento are:
- indexing frequency - how often are indexing jobs being sent.
- size of data being indexed - how much data is being indexed.
You can improve indexing time by making sure that you're using the indexing queue. It may also be useful to review this article on build time - What impacts build time.
The extension helps you control how many jobs to send to Algolia, but once those are sent, the build time may vary.