I didn't watch at the clock while creating this problem, but it lasted hours this time because it was so hard to eliminate many trivial side variations.
The pawn at f4 prevents the variation 1. Nc3 Nf4 (then Be6 wouldn't be possible anymore).
The pawn at e7 prevents variations where Rd6+ would win easily ...
Before composing my riddles most of the time I think about any theme I would like to treat: for example white wins by castling, by en passant, by smothered mate, like Nigel Short against Timman with his king, and so on. And then I build a chess problem around this idea ...
Thanks a lot for this insights.