Actually 4x 10% is 40%.

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.

THANKS! I was totally braindead when I wrote that. My 4% is actually the 8.1% you mention, and the way you phrase it makes it much clearer too :)

Suppose you spend 9 hours looking at the code and one hour actually editing code, and the latter is the only part that can be made faster (if we are to believe grandparent). Now, suppose this technology makes you 4x faster at editing code. Then you'll spend 15 minutes instead of one hour editing the code, and a total of 9 hours and 15 minutes. This is roughly 8% less time than you would have otherwise spent. Productivity, being the reciprocal of the time it takes for you to do the task in question, this is an increase in productivity of roughly 8%.

I read it as a 4x speed up of 10% of the work. So what took 10% now takes 2.5%. this leaves 7.5% remaining. Assuming that time is spent doing the same thing then it's 100/(90+0.925*7.5) which is ~3% increase in useful work. I'm sure this math is wrong.