I think it is safe to translate it as 'not steep'. But the actual use of slow in this style is when we describe a surface (or material) that slows. So I guess the steepness/gradient can slow so that is fine. Other situations could be the quality of the surface, for example clay tennis courts are said to be slow, while grass courts like WImbledon are fast. Or the race track is slow because of the type of cement, or the mountain bike trail is slow because of the mud.
So it is possible that slow in the story refers to the slope that is harder to ascend, not just for gradient but also for rocks, or slippery surface etc etc.
Side note: Why did London use slow? Well the sound S is repeated in words close by, (stammering gait up the slow slope toward the soft sky-line... 5 times!) and even LOW is repeated soon after, and even SLOW later. Also the words like limping, stammering, are all aimed at conveying this torturous and slow departure!