From a simplistic viewpoint, experience can be gained or nurtured by a good manager while speed is typically an inherit quality.
Conte made the mistake of being overly loyal to Cahill, starting him by name and captain's armband in the Watford debacle. Cahill is not in a good run of form to say the least. Chelsea really missed Christensen versus Bournemouth and Watford, conceding a ghastly total of seven goals without the Danish defender on the pitch. The 21-year-old displays an ice-cold composure and only gets better in his club's bigger fixtures.