Clicking on the "Pull" button and then looking at the log shows that SourceTree does a fetch and then a pull. This results in doing a fetch twice, because:

git pull runs git fetch with the given parameters and calls git merge to merge the retrieved branch heads into the current branch.

source: http://git-scm.com/docs/git-pull

The extra fetch has no benefit but does take extra time.

This issue can be resolved by either:

1. Removing the initial fetch and just doing a pull.
2. Replacing the pull command with a merge. The resulting fetch + merge is exactly what pull does anyway.