I recently started turning up at around 10 and leaving at 4:45 and I get just as much done as when I was in the office 9-6.

I've also started giving more fucks about work, which is amusing as I started showing up late and leaving early due to my lack of fucks.

So in my unsanctioned (and I'm sure time limited once someone complains) experiment with a sample size of one I can say shorter days definitely keep me more engaged and don't seem to slow down my work.