Lee Hinman over at writequit.org has an excellent post on working with log files in Emacs. By “log file” he really means “structured text” so, for example, most of his techniques work for things like JSON as well.
The post is fairly long so I won’t try to summarize the whole thing but it’s interesting how many built-in Emacs functions can be brought to bear on the problem. Everyday things like keep-lines, occur, and narrowing can make a big difference.
Hinman also talks about 3rd party packages that can help. One such package is View Large Files (vlf) that allows Emacs to gracefully handle huge files. That’s especially important for log files that can be multiple gigabytes in size. Read Hinman’s post for other useful packages and ideas for dealing with log files. It’s a very useful post.