What I Learned Building Alone
Working alone on something for the first time removes an excuse you didn't know you were leaning on: someone else's schedule. There's no meeting to wait for, no review cycle to blame for slowness. Whatever isn't moving is not moving because of you, which is clarifying and occasionally miserable in equal measure.
The upside is that feedback loops get very short. You have an idea in the morning and you're using it by the afternoon, wrong parts and all. You learn faster not because you're smarter working alone, but because nothing is buffering you from your own mistakes.
The downside is obvious enough that I won't dwell on it: there's no one in the room to tell you when you're wrong about something you can't see yourself. You have to go looking for that on purpose, which is its own kind of discipline.