Hi,
I would rather recommend increasing the post limit per iteration to a much higher number, say 200 first. Then if your server can handle that, it will be much faster. If there is a failure, the plugin will try to lower that automatically during the indexing, it will also notice you if the process fails many times, then you should start lowering that value.
If you have attachments to index, then those are indexed one-by-one regardless, as that is a very CPU heavy process.
The problem with the cron job is, that WordPress cron is not actually a classic cron job, it is triggered only whenever a page is opened and WP itself loads. That will not help much in this case.
I also recommend having the window visible on your desktop while indexing, Chrome and Firefox are known for throttling the javascript when a tab is not visible, and may delay the triggers.