The author didn't say that "performance didn't matter" as a starting assumption. He said it as a tentative conclusion, that specifically a small improvement in processor performance was not the most important factor in the user experience, or in adoption by tablet makers.
This is very clear in the consumer acceptance and sales figures of Apple iPad compared to the competition at the time, which was net books, with much more powerful processors. But netbooks ere hobbled by Windows. By slow boot times. By slow wake up. By slow hibernate. etc.... So the more capable processor was put into crappy context.
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