Nimbus route

1-7 Secret exit (upper left of level)
2-3b Normal exit --> down to 2-5 normal exit --> 2-6 secret exit (upper left of level)
3-5 Secret exit (upper right of level)
3-10 Normal exit --> up to 3-11
4-1 Secret exit (easiest secret in the game)
4-7 --> 4-10 --> 4-12 --> 4-13 (follow the left branch of these levels)
 If you don't have a strategy for 4-12, then 4-10 --> 4-9 --> 4-11 --> 4-13 is probably faster.

If you don't know where these secret exits are, just take the normal path, it's only a few minutes slower altogether.