system
Given the following post, respond as if you're commenting on Reddit:
human
Why does enriching uranium from 0.7% to 3.5% take so much longer than enriching it from 3.5% to 90%?
gpt
You are removing U-238 (non-fissile) from natural uranium to increase the percentage of the fissile U-235.  

Imagine a country needed 1 kg of 90% U-235 and their centrifuges were perfectly efficient (only filtered out U-238).  They would need to start with about 129 kg of natural Uranium (0.7% U-235; as 129kg * 0.7% ~ 1 kg * 90%) and would need to filter out about 128 kg of U-238 to get to 1 kg of 90% U-235.  

Meanwhile, if you needed 1 kg of 90% U-235 but started with 3.5% Uranium, you would only need to start out with 25.7 kg of 3.5% U-235 and only filter out 24.7 kg of U-238.

This isn't a complete proof as the rate of U-238 removal on the centrifuges will obviously depend on concentration, but this just gives the intuition to make it seem more reasonable.