Is it always faster to get data from the cache?

No! When caching, there is a balance between speed and size. Let's use another example to illustrate this. Say that you configure your web browser to cache 20MB of data. By doing this, the browser would almost never have to actually download anything from the Internet (unless the content had changed, of course); everything would be in the cache. But, your browser would require a long time to search through all the data in the cache. In this case, it's a lot faster to get the content from the Internet than to search through such a massive cache!

Is the browser's disk cache secure?

Definitely not! Cache files are accessible and visible to anyone. Another person can view your cache files to see what web sites you have visited.