The formatted JSON looks pretty, but on large files it takes a long time. The addon prints its progress as a percentage in the tab title, but it is still very very slow. You cannot search within the file before it is finished rendering.

JSON Lite will automatically start formatting any JSON document. Since that can take a long time I would prefer if I can activate it only when I need it. Another addon, Basic JSON Formatter, does so by adding an icon in the address bar on JSON documents.