Cron - All nodes are indexed at once

The cron limit that can be set via the UI only determines the batch size in which items will be indexed during cron. However, multiple such batches will be indexed if time permits – this is determined by the (hidden) cron_worker_runtime setting in the search_api.settings config object. The maximum run time defaults to 15 seconds, but setting this to 1 or even 0 will switch the behavior to index only a single batch per cron run, so that’s probably what you want to do.