The Debug Toolbar will only display itself if the mimetype of the response is either text/html or application/xhtml+xml and contains a closing </body> tag.
Be aware of middleware ordering and other middleware that may intercept requests and return responses. Putting the debug toolbar middleware after the Flatpage middleware, for example, means the toolbar will not show up on flatpages.
The Debug Toolbar depends on the jQuery library. If jQuery isn’t already loaded in your page, the toolbar loads it from the Google Hosted Libraries CDN. Your browser will keep it in cache, allowing you to use the toolbar even if you disconnect from the Internet temporarily.
If you want to use the Debug Toolbar without an Internet connection at all, or if you refuse to depend on Google’s services, you should include your own copy of jQuery on every page.
The Debug Toolbar is designed to introduce as little overhead as possible in the rendering of pages. However, depending on your project, the overhead may become noticeable. In extreme cases, it can make development impractical. Here’s a breakdown of the performance issues you can run into and their solutions.
The Debug Toolbar works in two phases. First, it gathers data while Django handles a request and stores this data in memory. Second, when you open a panel in the browser, it fetches the data on the server and displays it.
If you’re seeing excessive CPU or memory consumption while browsing your site, you must optimize the “gathering” phase. If displaying a panel is slow, you must optimize the “rendering” phase.
The SQL panel may be the culprit if your view performs many SQL queries. You should attempt to minimize the number of SQL queries, but this isn’t always possible, for instance if you’re using a CMS and have disabled caching for development.
The cache panel is very similar to the SQL panel, except it isn’t always a bad practice to make many cache queries in a view.
The template panel becomes slow your views or context processors return large contexts and your templates have complex inheritance or inclusion schemes.
If the “gathering” phase is too slow, you can disable problematic panels temporarily by deselecting the checkbox at the top right of each panel. That change will apply to the next request. If you don’t use some panels at all, you can remove them permanently by customizing the DEBUG_TOOLBAR_PANELS setting.
By default, data gathered during the last 10 requests is kept in memory. This allows you to use the toolbar on a page even if you have browsed to a few other pages since you first loaded that page. You can reduce memory consumption by setting the PANELS_CACHE_SIZE configuration option to a lower value. At worst, the toolbar will tell you that the data you’re looking for isn’t available anymore.
If the “rendering” phase is too slow, refrain from clicking on problematic panels :) Or reduce the amount of data gathered and rendered by these panels by disabling some configuration options that are enabled by default: