Being "swept away" in sub-light requires space to be viscous.
While oddly named and with slightly-fudged properties, it's fairly clear that "sub-light speed" refers to relativistic speeds.
Until the last episode, where consistency goes out the window. (Lead is stranded 100 years in the future, only to be picked up anywhere from 30 seconds to a few days later; her friends have only aged 20 years at the very most.)
If they followed her in sublight, which they would have had to, they wouldn't have aged that much either.)