He is famous for his fast climbs, holding speed records on several routes including the Matterhorn in Switzerland and the Cima Grande di Lavaredo in Italy.

[3][4] Arnold was already known in the climbing scene before 2011, among other things for the speed record of 1 hour and 35 minutes on the west ridge of the Salbitschijen.

In 2010, he succeeded in the first winter ascent of the Torre Egger in Patagonia with Stephan Siegrist [de] and Thomas Senf.

In April 2013, he and David Lama made the first ascent of the central headwall on the 1500-metre-high east face of The Moose's Tooth in Alaska.

[7] In 2019, he reached the summit of the north face of the Cima Grande di Lavaredo, in the Dolomites, by climbing the Comici-Dimai route.