My colleague veteran pilgrims have aptly stated all the "normal" speeds above. My take on it is, what I call the '345 rule' (of thumb). That is 3 kph on most dedicated uphill or downhill segments, 4 kph on rolling or mostly flat terrain, and 5 kph on totally flat terrain, like the Meseta.
On very steep inclines, like coming out of St. Jean to Orisson, the pace does slow to about 2.5 km per hour, for most folks. It takes about 3 hours to walk up the hill to Orisson, 8 km from St. Jean Pied de Port. THAT is why so many of us like to stay the first night there.
As stated above, many Europeans can do in excess of this pace, and covering 6 or more kilometers each hour is not unheard of. Although WHY, I still have not figured out...
Downhills can take as long as uphills, especially with a rucksack, or on slippery or uneven surfaces. Those of us, on the "south side of 60," will typically use our walking sticks to ensure that gravity and inertia do not propel us downhill at too great a speed, and off the trail into a ditch, or worse. When I was younger, I could run down hills, even with a rucksack on, taking advantage of the added pull of gravity to move faster. Now, I gauge the benefits of being a human 'snowball' rolling down a mountain path, against the likely cost of a hospital stay to mend my broken body...
Besides, the faster one walks, the less one experiences what the Camino has to offer. Some pilgrims have valid reasons for needing to process down the road at a quicker pace. I respect this. But, in general, I always advocate taking one's time and enjoying the journey. I have learned that the journey IS the destination. In fact, my wife framed that missive when I did my first Camino, and it now hangs in our home.
I hope this helps.