Starts off painfully slow. First upgrade you can afford is extra doors. Realistically one would expect it to be "instead of 1 door spawning, 2 doors now spawn", though that only happens like 50% of the time, for some reason.
No real clue as to why it only spawns an extra door randomly.
Main thing is that the early game is again, painfully slow. You break a door, wait 1-2 seconds, break another, each giving $1. Eventually you get $8 and you occasionally spawn an extra door, but still with the 1-2 second respawn timer. Its just... slow.