I've used both, and I wouldn't describe the time savings as significant.

Compare the case study of Yang, who runs either

    git checkout neighbour-fix
    git checkout -b yang-work
    <time passes>
    git checkout neighbour-fix
    git pull
    git checkout yang-work
    git rebase neighbour-fix

or

    arc patch --diff D12345 # arc's a CLI tool for Phabricator
    <time passes>
    git fetch
    git rebase -i HEAD~2
    <goes into phabricator and finds the new commit hash for D12345>
    <pastes it into vim in place of the original commit hash>

From this, it's not clear to me that Yang is getting the better deal if he uses Phabricator. Certainly, he has to enter fewer commands, but he also has to leave the terminal to go copy a commit hash from Phabricator, and then paste it into an interactive rebase session. If given the choice, I'd choose the PR workflow over the Phabricator workflow, based on not switching to the browser and not needing to copy-paste things into an interactive rebase. Phabricator and arc handle deep stacks of diffs more nicely, but that's rare enough that I'd rather optimise for the common case.

Apologies if I've misunderstood exactly which commands Yang would run in either case.