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 the

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.