The major gotchas is building the indexing engine. Once you have that in place, getting the commit information takes milliseconds. The only time consuming thing is extracting the from and to blobs from the repo. However, that is a one time hit, since you can cache the blobs.

For the diff, this is done in realtime with https://github.com/Microsoft/monaco-editor

Also my product isn't really dependent on GitLab or GitHub. It works in standalone mode as well.