You can have the tool start by writing an implementation plan describing the overall approach and key details including references, snippets of code, task list, etc. That is much faster than a raw diff to review and refine to make sure it matches your intent. Once that's acceptable the changes are quick, and having the machine do a few rounds of refinement to make sure the diff vs HEAD matches the plan helps iron out some of the easy issues before human eyes show up. The final review is then easier because you are only checking for smaller issues and consistency with the plan that you already signed off on.

It's not magic though, this still takes some time to do.