My Nexus 5 is so slow and clunky in Battery Saver mode that I seriously wonder if it actually helps. Taking a minute to do something in Battery Saver mode isn't necessarily more efficient than spending 5 seconds in normal mode.
I've found the exact opposite... by turning on Battery Saver mode, it disabled all the extraneous animations and transitions and actually made my old Nexus 4 feel responsive again!
Under the developer options section, you can disable animations.