No doubt the three shortest playlists are:
- Vehicle Rally - Uplift Reserve, Kizingo Boulevard and Kikowani Station on Normal
- Arena Battles - Alpha Site and Coastal Highway on Normal
- Hoofin' It - Tayari Plaza, NMPD HQ and Data Hive on Legendary
If you'd prefer you could rush through the New Mombasa Streets playlist. This will be on legendary however. This may be easier seeing as there are less enemies. - Thanks to Hey Im Swat for the pointer.
To save time you may want to complete the campaign on one of the easy, normal, heroic or legendary playlists on your first playthrough. Or you could just do the easy or normal campaign playlists if you'd just prefer rather than doing Hoofin' It.