Corona's default render mode is progressive this means that it'll start shooting rays and immediately display the results on screen so you get an image almost right away, this doesn't mean the render is finished, it'll just keep shooting rays and improving the quality of the image until you tell it to stop.
You can't just magically increase the quality with a slider in any renderer without it costing more time. A higher quality means that you need more samples which needs more time/computing power.
What I would suggest is that you set up your scene, reset ALL settings back to default then press render and don't touch anything for at least an hour. That should give you a decent quality image in almost all cases.