Super cruise dynamics?

[Ism]

So I jump into super cruise from a next to a star and accelerate to 1c in just a matter of a couple seconds.
When I jump into super cruise from an outpost, station, or accelerate after planetary exploration it takes around 30 seconds to achieve 1c.

What other than the gravity well interferes with speed?

This slow acceleration is fairly constant regardless of the planet size or if the planet has moons.

 

[Loskene]

Distance to large sources of mass, the FSD seems to have a few gears it changes into depending on how close you are and how massive the thing is. It's stuck in first gear doing 100km/s next to the dropout barrier of anything, but it won't hit 1C unless it's far enough from anything to "gear shift" quick enough up to it.