I have observed that while many science students aim to finish within 3 years, most arts and humanities students give themselves up to the full 4 years. I know very few arts and humanities phds who have submitted within 3 years. If this is indeed a general trend, is it because an arts phd requires more 'cogitation' time, because arts students are lazier, or because they don't tend to have as much day-to-day structure and guidance as their counterparts in the sciences?

To be honest, I don't know many science PhDs that stick to the 3 years either.

I don't know anyone who has finished in 3 in the sciences. From what I've encountered, most scientists do 3 - 3.5 years doing bench work and then however much longer to write, I'd say probably 4 years in total.

I think at least part of this is funding: only 25% of applicants in my subject (English) get AHRC funding, and that doesn't take into account non-EU students who can't even apply. One reason A and H students take longer is because the majority are working part-time and self-funding. I think it's a miracle we manage to complete in four years, frankly.