The Covenant get Vasari Phase Stabilizers which is awesome and fits the Lore vs the UNSC pretty well.
But the Forerunners have even better slipspace technology (Halo: Silentium when the Librarian left the Milky Way galaxy to find a group of ancient Forerunner mutineers).
My best idea is that Forerunners could build portals instead of Phase Stabilizers and those portals would allow instantaneous travel to other portals.
It would fit the Lore better if the Forerunners are also just plain faster when they Jump but that would probably be overpowered even if it was a high-tech research option.