Based on Git, a distributed VCS
Git, on which GitLab is built, is fully distributed. Every user has a complete copy of the repository, allowing for much faster access to history, easier branching and an overall better experience.
A branch in Git contains the entire history that preceeds it. It's also created or moved towards instantly and easily shared.
Git is fast
Git is really fast. Performing a diff, viewing history, committing and merging changes, obtaining any revision and any form of branching is very quick.
Git has smaller size requirements
A single repository in Git is typically a number of times smaller than the same repository in SVN.