At 10% it's approximately the same. But say 50% instead.

If it takes 1/2 the time to do something, you can do double the work in that time.

If on the other hand, you can do 1.5x the work in the same time, it means you've made it use 33% less time than before.

With 10% less time you're taking 1/0.9 = 1.11111x more work in the same time.

Not exactly, "doing 10% more work in the same time" means needing 9(.1)% less time to do the same work (100 / 110 versus 90 / 100).

Needing 10% less time for the same amount of work means you can do 11(.1)% more work in the same time.

At it's heart they seem to be asking kind of a latency vs. throughput question. The answer is frustratingly "it depends" but for some real world Rails benchmarks, Shopify has published quite a few benchmarks including the ones in the linked article.

The percentage is applied to two different things. One for work and the other for the time.

It took sometime to me too.