What I pick up in the first week
Component boundaries that stopped making sense three features ago, state that turned into prop drilling, and forms where validation lives in four places. Then the states everyone skips under deadline: empty, loading, partial, error, and what the screen does when the request simply never comes back.
Where the money actually goes
Usually rendering rather than features. Bundles that ship the whole library to render a table, effects that fire in a loop, lists that re-render on every keystroke, and layout that shifts after the fonts land. These are measurable before and after, which means you can see what you bought.
Where I stop
At the API. If the interface is slow because an endpoint returns four thousand rows, or awkward because the payload does not match how the screen is used, I will say so rather than building a workaround in the client and calling it done. Papering over a backend problem in the frontend is how you get a second problem.