The drawback with long time to iterate is you have to be in flow to do things. That's non-negotiable in environments with a lot of meetings.

There's also a "sketching" element with faster iteration. On slower iterations, you're engineering. The engineering skill is often underdeveloped but sketching is more valuable without a dedicated designer.

Also some things are poorly documented, often things like UI and browser code or legacy support for say, older Android OS. So even if you have your calculations right it ends up wrong.