If you are seeing a folder icon with a question mark at start up but it still boots up, perform a full backup of your device immediately. Unless, you are booting from an external drive which could explain a momentary delay in locating the Operating System this icon usually indicates your Hard Drive is about to fail.
There are a few things you can do about this, such as creating an external bootable drive especially with older Intel Macs running earlier macOS. A free trial version of Carbon Copy Cloner will allow you to create a Bootable Clone that will run your Mac and be a duplicate of everything you have now.
More info if you're interested.