I think the biggest problem comes there for a couple of reasons:
- By the third circle, you really don't have enough of them memorized to go quickly. You're still having to manually search, find and check your solution.
- I normally spend between 30 and 60 minutes a day doing tactics, which means the first circle is good, the second circle tends to go over the hour, and the third circle is rarely finished before 90 minutes.
In future seven circles exercises (including this one) I'll have to be pretty conservative with the number of puzzles I try to do per day - especially in the early circles - because by the third circle you don't want it to get so difficult that you feel like quitting.
Anyhow - I'm about halfway through my second circle of Lou Hays' book and I'm already having concerns about the time it takes.