You will notice two fields named Scale and Zoom in the Geometry table on the Flame tab. These can both be used to make the image zoom in or out, but do different things internally. This article clarifies the differences and offers recommendations on their usage.
When fractal flames are zoomed in, they will degrade in appearance and become more speckled/noisy.
Scale zooms the image without a corresponding increase in quality. For images with a small scale value, or a quality value that is already large, this shouldn’t increase the noise by much. However, for the inverse situation, you might need to increase the quality a bit to compensate for the increased scale.
Zoom does what scale does, however it greatly increases the quality to compensate. The actual value internally used for quality is displayed on the status bar as “Scaled quality”.
In most cases, using zoom is overkill and will result in very lengthy renders. You might have a low value for quality, but after zoom increases it behind the scenes, the final quality used will be quite large. Delays are further compounded because after each edit, the thumbnail updates. Even though they are rendered with low quality, the usage of zoom greatly increases their render time, which can lead to a sluggish UI.
In conclusion, if you need to zoom in, use scale and manually increase the quality. Only use zoom if the increase of scale resulted in unacceptable noise. The two can also be combined if needed, but it shouldn’t be necessary.