> Building the wrong thing is expensive. Building pieces that don’t fit together is expensive. Avoiding those mistakes requires investing time in some sort of planning activity.
The promise of "agile" is, IIRC, faster turnaround and iterative development. Yes, the 'wrong' thing is expensive. Getting something basic in someone's hands in 2-3 days to get some feedback is usually more productive than days of figma then 'sign off' then work work work then putting it in hands and getting back "this is wrong" (missing data, etc). Sometimes those things can be found in 'design-only' sessions, but I've rarely seen it happen where lots is found there, then things are implemented perfectly afterwards not requiring any further changes.
Much depends on the size/scope of the work, and I don't think there's one-size that fits all, but the ticketization of work process as teams grow pushes towards "one true way", which then seems to paper over a lot of cracks that don't easily show up in burndown charts.
That 15 or 30% might be the time ultimately taken on thinking/planning/etc of a specific issue, but too often it seems there's an idea of doing more up front "gets it done" and the notion of fast feedback/iteration cycles is glossed over, and you're expected to "get it done" - "all the planning was already done".
When doing dev-type work, I always end up needing to iterate with endusers/stakeholders, and there's little 'up front' that could have been planned to avoid some of that. It may be the nature of the people/teams I've worked with, though.