When John and Jade were going through the Yellow Yard, John asks if they could go any faster to which Jade replies that the fastest the ship can go is just sub the speed of light. When does she say this?
JADE: im just trying to estimate our arrival time
JADE: based on our current velocity, which is about as close to light speed as i can make it go
Then in the next one (what do you call these things? panels? screens) Jade gives more of an explanation, which I mostly can't follow because I haven't read Homestuck, but it includes the following line:
JADE: and once we leave the suns domain, our travel is limited by the speed of light, like everyone else!