Why are you starting the clock at the time when you already have a "high level plan that you are extremely familiar with"? I think it's fairer to start from "I received a bug report/feature request" or similar.

Also, haven't you ever had a situation where the prompt you started with ends up being longer than the final code diff? Perhaps a subtle bug that's hard to describe/trigger, but ended up having a simple root cause like an off-by-one error?

Also also, coding agents are infamous for generating way more code than is strictly necessary. The 2000 lines of code that the agent generated may well have been only 200 lines had you written it yourself.