By 4x "increase" in 10%. he means you'd become 4x faster at things that take 10% of your time currently (the other 90% corresponds to the "coming up with soluti

By 4x "increase" in 10%. he means you'd become 4x faster at things that take 10% of your time currently (the other 90% corresponds to the "coming up with solution" / "wtf" moments). That means for a job that takes 10 + 90 units of time, the 10 section would be done in 2.5, for a total effort of 92.5 instead of 100. That's an improvement of 100/92.5 - 1 = ~8.1% in productivity.