fastest way to trigger a flow (to avoid hour delays)

I’m looking to avoid any 1-hour delay in triggering a flow. (I do not want a campaign. Must be automated flow.) What’s the most instant way to trigger a flow so the person signing up gets the email in the absolute fastest time with minimal delay?

I know a segment triggered flow can take up to one hour (not acceptable for the situation I am working on). 

What about list triggered flow? Are those immediate?

Are there any other options for triggering FLOWS instantly w/o delays? (Up to 5 min. would be acceptable. But no more. Needs to be instant.)

Have read this: https://help.klaviyo.com/hc/en-us/articles/115005233488-How-Segments-Update#what-do-we-mean-by-real-time-1

Have looked here: https://help.klaviyo.com/hc/en-us/articles/115005253208-How-Often-Integrations-Sync But this doesn’t apply as I won’t be using integration. People will fill out Klaviyo form to get added. No third party involved.