Vim sometimes chokes on long lines, and very rarely on long files. Both of these are caused by the plugins and settings you have enabled though, not by how the editor works.
If you have a plugin that scans through every line in a file (yes, some are this stupid), a large file will take ages to load. Some syntax highlighters that use inefficient regexes have a similar problem, but with long lines.
You can test this by opening your giant file with `vim -u NONE large.txt`. That disables your .vimrc.
