A few notes collected from 'The Road To Excellence' - An interesting book edited by K. Anders Ericsson.
A more direct claim about the necessity for intense preparation was made by Simon and Chase (1973). They found that about 10 years of preparation was necessary to attain an international level of chess skill and they suggested similar prerequisites in other domains.
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Even when individuals have access to a similar training environment, large individual differences in performance are still often observed. Furthermore, amount of experience in a domain is often a weak predictor of performance. Rather than accepting these facts as evidence for innate differences in ability (i.e, talent), Ericcson, Krampe, and Tesch-Romer (1993) tried to identify those training activities that would be mist closely related to improvements in performance. Based on a review of a century of laboratory studies of learning and skill acquisition, Ericsson, Krampe, and Tesch-Romer (1993) concluded that the most effective learning requires a well-defined task with an appropriate difficulty level for the particular individual, informative feedback, and opportunities for repetition and corrections for error. When all these elements are present they used the term deliberate practice to characterize training activities.
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They found that concentration is the most essential aspect of deliberate practice. Deliberate practice in music is consistent with the critical role of concentration. Master teachers (Auer, 1921; Galamian, 1962) argue that full concentration is essential and that when it wanes the musician to rest, because practice without full concentration may actually impair rather improve performance (Seashore, 1938/1967).
An analysis of practice sessions (extracted from the diaries of the expert musicians) is consistent with the requirement of concentration. After about an hour of practice in the morning when the ability to perform most complex cognitive activities has been found to be the highest (Folkard & Monk, 1985). There is also indirect evidence for the effortfulness of concentration through the additional need for rest among expert performers. Ericsson, Krampe, and Tesch-Romer (1993) found that the higher levels of practice were associated with longer duration of sleep, typically in the form of afternoon naps. This pattern of napping and resting has also been found in other domains expertise, such as sports.