The reindexing operations can take some time, particularly if you have a a significant amount of products in your catalogue. This is because Shopify has to process the data before sending it to Algolia and we have no way to make this part of the process faster.
We recommend that you rely on real-time indexing to keep your data synchronised with Algolia, so manually triggering a reindex shouldn't be necessary under normal circumstances. It is also important to note that when you are running a reindexing operation the real-time indexing is suspended, and therefore, you will experience a delay on updates and data synchronization.