I have two screenshots to contribute to this thread.
Disclaimer: I don't know if AC Year count is actually a turn for a year or not. I can't find turn timer anywhere. cuz if it is then it been 70 turns until I built Onyx throne..? Or 140? Dunno!
My current 1.4 game.
The very first turn of game, AC 165. Gaze upon the splendor of my riverside village which eventually became a Town!
I name it Rose! My subjects rejoice!
And it is now turn AC 235.
A onyx throne has been constructed! For my rebellious subjects to learn how weak they are and prostrate before my might!
This is my third city I built! My second is a conclave in a isolated but protected location.
Zanber Fortress has selected Strike Garrison, Prison, and Onyx Throne. And it is built next to river which boost it's food limits while the tile yields the fortress is built on has exactly 5 grain and 3 production and 0 essence! As you can see, the fortress have 115 production. It's growth is 5. I did get lucky there and shaved a whole bunch of turns off because of a temple of forgetting. Its my first time getting it to be honest. Otherwise I would've built 1-3 extra constabularies for the fortress. Oh and it is currently production growth but its done now because it got the Onyx and is now back to building any leftover upgrades and soldiers for armies. This fortress alone has produced five armies so far and the said armies is protecting all my fronts and subjugated Imperia Wildlands to the south and became part of my territory.
While I am aware that Essences is like OMFG. But dude, Riverside fortresses is way more important if you want to have Onyx up fast. Because of the Pier line improvements boost food income plus the final upgrade the watermill also boosts production and the Zanber fortress did manage to nab The Great Mill wonder as well. And is next to forest. I chose that location over settling on 3 food and 3 production and 2 essence but that tile isn't next to river and forest but is however right next to a swamp which means horrible production.
It cranks out a light plate armor soldiers every two turns with 115 production. The time that onyx thrones require is just about right I think. If it came even faster, my soldiers will be crashing headalong into even more powerful monsters before they're ready and interior still isn't fully stabilized with bunch of quests to be done still.
Oh and Vergas cast tremor on my Caravan. I thought that got fixed in 1.4 Or was it just for wither?
If you settle fortresses in low food but high essence tile locations, just suck it up like a man and acknowledge that the fortress won't get level 5 any time soon but late game it will its the price you pay for grabbing delicious high essence tile otherwise it would be too overpowered and it at least give times for opposing factions to squash your fortress before it got that ultimate level 5 stuff up. But hey, that 1 food 5 production and 5 essence tile means you can build the fortress upgrades very fast and switch to cranking ubar enchanted soldiers right away just build another fortress in different location which will be intended to build a onyx throne in relatively fast time.