I can't remember if it is in the manual but I have plans to change construction time for many Wonders based on nearby resources. Mostly Stone or Marble, but a few others like Copper (for Colossus).
Since it came up, I'll keep in mind creation time for Tomes as a possible Enabled Policies effect.
But I think the 25 turns is about right for "great works", meaning wonders, epics and various artifacts like tomes (I was using that term long before BNW so I'll keep using it). If an item isn't really "great" enough to justify the time, then that is an area for balancing. On Arcane Towers specifically, it seems to me that players have evaluated them as worthwhile even with the long build times. Does anyone even use the "Occupy Tower" ability?
There is room for discussion around game speed adjustment for build times, but that's a complicated issue. Partly, it's challenging to mod in. But there is also a "what do we want game speed to mean?" question. My thinking was that an epic game would not only have more turns, but also more "generations" of GPs, more wonders, more everything. A quick game would be more focused with a smaller number of all of these. (Even if you try to "scale everything", an epic game is still fundamentally different in that you can wage many more wars.)