Normally a PHP application starts from scratch on every request: read the configuration, assemble the services, register the routes, do the work, throw it all away again. That is remarkably robust, because no request can leave anything behind for the next one, but it costs time on every call.
Octane turns that round. The application is started once and stays in memory; requests are served by workers that keep running. The start-up disappears and the response time drops noticeably.
What you take on in exchange
The robustness was no accident, it came from the restart. When the application stays in memory, whatever a programming mistake leaves there survives too. A value that accidentally persists between two requests can mean that user A briefly sees user B’s data.
That is manageable, but it demands care in places where none was needed before, and it applies to third-party packages as well. So in our projects Octane is a deliberate decision taken after a measurement, not something that runs along by default.