) Propulsion method. The 'effective' speed is slow, but note that's actually not for the FTL so much implied as the 'sublight' approach or other factors. Its implied the actual transit may be quite fast. What's more, the main reason they have to pull this off outside a system seems to stem largely from the means of navigation. If they have something else to navigate by (EG a psychic beacon of sorts like the Genestealers, or someone in active communication with the fleet via the hive mind, which is IIRC better than Astrtelpathy WRT communication) it could be argued to be much more precise and even faster.Whilst it is true tha tthe Tyranids are constrained by sublight speeds whilst within the borders of a planetary system, they are capable of far greater velocity when traversing interstellar space.
..
...Narvhal is almost completely defenceless, with little in the way of bio-weaponry and a comparatively thin protective carapace. A cluster of monofilament spines on the Narvahl's bow enable it to interpret a wide range of sensory input, including an unbelievably broad spectrumof gravimetric signals. Using these senses, the Narvhal can detect planetary systems at incredible distances. It can then harness that system's own gravity, creating a compressed space-time transit corridor through which the Narvhal, and nearby vessels, can cover vast distances. This method cannot be employed near to strong gravitational forces, as they drown out the more subtle traces that the Narvhal uses to navigate. As a result, a Tyranid fleet must rely on more conventional propulsion in the final approach, in some cases slowing their arrival by years, or even decades. Whilst this combined propulsion method is slower than Warp travle, it is infintiely more reliable...
The 'side effects' I talked about. That sort of thing is bound to be troublesome, nevermind disrupting attmepts to reach the system given the implied 'strength' of the gravitic fuckery (it takes far less to interfere with hyperdrive, arguably.) and the natural disaster disruption would be a good cover for Tyranid assault.The Narvhal's manipulation of a star system's underlying forces is not always without side effects. A prey planet will sometimes be subjected to earthquakes, solar flares, tidal waves and other natural disasters in the time between the Narvhal casting its gravitic snare and the Hive Fleet actually arriving. This only benefits the Tyranids' efforts, guarnateeing as it does that the defenders of the target world will still be wrestling with planetary disaster when the swarm arrives in orbit.
That alone kinda adds some interesting wrinkles to the discussion, though even with warp travel I can think of some cases where they circumvent the 'Behemoth' like approach (even coming out close to the surface of a planet in one case and attacking within seconds or minutes in one of the Cain short stories.)
