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I don't think being doped makes you descend faster.
"Fun" story:
In the 1960 Tour, Roger Rivière was second to the Italian Gastone Nencini, a rider he planned to beat by tagging along with him in the mountains and then speeding away on the flat. The problem was that Nencini was lighter and a better climber and that he was such a fast descender that, in the view of another French rider, Raphaël Géminiani, "the only reason to follow Nencini downhill is if you've got a death wish."
Rivière was able to stay with Nencini on the climb to the Col de Perjuret, as the pair crossed the summit together. Then came a series of descending zigzags. Nencini took the perfect line and Rivière, trying to match him, overshot a bend, fell into a ravine, and broke his back. There he was found by his teammate, Louis Rostollan.
Rivière quickly passed the blame for his fall and his broken back on the team mechanic, accusing him of leaving oil on the wheels and the brakes for not working. The mechanic was outraged, and the doctors soon found the real reason – that so much painkiller was in Rivière's blood that his hands were too slow to operate the brakes. He had taken a heavy dose of the opioid painkiller dextromoramide (Palfium), to help him stay with Nencini on Col de Perjuret. Rivière later admitted to being a drug addict, telling a newspaper how he had doped to beat the world hour record, and admitted downing thousands of tablets a year.
Except Froome's break wasn't 30km it was 80km.
I didn't think he was juiced then and don't now but think whatever rider made that breakaway to go into pink would have faced the same disbelief. A rider way off GC doing a 30k break on a mountain stage isn't the same thing.