I am sad to announce that the callendar slowing is not quite flawless.
My men spend way more time doing "eating" and "drinking" than in normal time, resulting in a fortress with way less work done "per tick". Also, while eating, they get thirsty, and hungry while drinking (because it's a 'per tick' increment thing) as usual, so they sometimes even eat>drink>eat>sleep>drink. It can become a bit infuriating.
While it still allows for more work per season, it feels way more like a waiting game than it usually does.
Added to that weird things (some might not be completely related or at all).
Sometimes, they will take a long time between tasks, or in the middle of one (storing suff into a new bin is a noticeable one).
My hunter/bow man has equipped all his stuff, but refuses to go get his arrows (even when enabling only arrows in the "ammunition" screen). Might very well not be linked to the 1/8 calendar, but it never happened to me with dwarves and bolts when doing the exact same thing.
In the end, he can't fight, and won't hunt, saying 'no job'.
I was so happy with this calendar speed thing, because I usually don't plan well enough and get swarmed before I am ready, but it is not making it fun to play for me with 1/8.
Was somewhat OK with 1/4, less noticeable downtimes, but I suspect it's just because it's half the "extra" time spent eating.
Anyway, this is not based on any science, just observing my small band and finding that many tasks. Might be wrong, and just the game deciding to make them do these things as intended, so I would like to know about other people's impression about 1/8 calendar speed.
EDIT: Scratch all the above, Firefox (yeah, I know) is using 1.6GB of ram, making the whole computer react a bit odd to some input, which could have been what made the game act odd.
I'll try again tomorrow, without that 'thing' taking my computer hostage.