In some sense, standup is a nocebo, a mood killer for me. I am there, in the morning, ready for work, my mind firing up, geared towards working the problem, but no, I have talk and listen about something unrelated. It's like going to a movie and having to suffer the trailers for other movies first.
(There was also this funny psychological research that showed that when you tell someone you will do something, you're less likely (less motivated) to do it. Not sure if true, but if it is then standups alone have caused billions of loses in productivity.)
In any case, I think the discussion about individual productivity is somewhat sidetracking the core issues. What I wrote applies to the whole team as well. In my experience, the best design decisions come from individuals working alone, understanding the problem, and then these design decisions are vetted (and possibly changed) by everyone in an open discussion. This cycle can happen on almost any timescale, depending on the nature of the problem. So one doesn't need to be in an ivory tower for this to work.
Timeboxing as such is not a bad idea. But I think conflating it with "being done" is just wrong.
