The race narrative on the Pez site gives an indication of just how demanding is the climb to the finish: "Billed as the hardest day on the hardest climb of this year's La Vuelta a España, in fact it’s been billed as the hardest climb anywhere in any stage race." Its winding track of a road is steep, of course. The weather is commonly abominable, thought yesterday it was only cold, not cold, wet, and windy. And the altitude is way up there. The summit is 1,570 metres (5,200 ft) above sea level, from bottom to top, racers had to ascend 1,248 m (4,090 ft), and near the summit the slope is over 20%.
Today, a Pez article by Ed Hood gives some nice photos, including these. Note the helicopter in the background of the first one. The second one shows the race leader coming through on his own.