The really hard extra levels will not start appearing until the third/fourth world.
And skipping any single level in any world doesn't count.
At the end, if completing 50 levels took me 15 hours, it's because there are only a few levels that delayed my progress. Surprise, the hard ones.
When I talked about the time to complete the game, I was talking from the perspective of a completionist not abandoning a world until having completed every single level and without using a single clue, tip.
If doesn't mind if you complete 10 levels in a row taken you a few minutes, if it appears a hard one, delaying your progress two hours.
And believe me, these levels will appear :)
However this is a game where "skill" will have a huge impact on the time to 100% complete the game.
A genius would probably be able to 100% in about 10 hours, whereas some players will need more than 100 hours.