Not to mention, that bosses when defeated will leave behind a Shortcut, which means there's no "Time Spent Running Back" and killing the last boss will auto-raise you if you're dead.

The only time you have to run back, is when there's a wipe.

EDIT: Well, ok. There's the Aetherochemical Research Facility, because there's actually 2 bosses at the end and getting killed on the first means a somewhat lengthy run back to fight the 2nd.