300,000 lines of code doesn't seem unattainable for a single developer in a year. If I'm in the zone, I can easily bang out 5-10k lines of code over a weekend if I know what I want to write.
But that's the thing: a lot of development involves not writing any code, as you re-think your abstractions, plan the architecture of the next bits you'll write, debug what you've written, etc.
So I don't think we're necessarily talking about speed when we say that it's harder to read code than to write it. I think reading -- and truly understanding -- code (especially when it's someone else's, or even yours, that you haven't seen in a long time) can require quite a bit more mental effort than writing code.
But that's the thing: a lot of development involves not writing any code, as you re-think your abstractions, plan the architecture of the next bits you'll write, debug what you've written, etc.
So I don't think we're necessarily talking about speed when we say that it's harder to read code than to write it. I think reading -- and truly understanding -- code (especially when it's someone else's, or even yours, that you haven't seen in a long time) can require quite a bit more mental effort than writing code.