It depends on the frame of reference. Obviously we're talking about a solar reference frame here with respect to linear velocity (it's the only other object),

It depends on the frame of reference. Obviously we're talking about a solar reference frame here with respect to linear velocity (it's the only other object), but it will also be rotating. If the reference frame is initially rotating at with your orbit or a closer one, "speed up" is accurate. If it's rotating with some more distant orbit, "slow down" is accurate. This, by the way, is one reason why physicists tend to just say "accelerate" in both cases: then, it's technically true for any external reference frame.