Think of it this way: As long as you don't spawn on a continental divide, getting the extra 10% yields is essentially automatic. It directly reduces the AI's relative yield bonuses. It straight-up reduces the difficulty level with no action required by the player.
Throw in the bonuses from relics/sculptures, and that excellent unique neighborhood, and she can absolutely explode in the mid-game.
That is not anywhere close to being overpowered though.
I'm not saying its a bad bonus, but its absolutely nowhere near "OP".
The problem with this ability is that it is a very slight increase in all yields, especially early for the early game where you want these sorts of abiltities to matter.
The early game is defined by the player being behind the AI in every way (assuming Deity level play), and there isn't much you can do about it as a general vanilla civ.
That is, unless you have some edge that allows you to reach a certain breakpoint fast, and exploit said breakpoint.
A typical example of this is warfare - if you have some very strong early warfare bonus (Gaul or Nubia for rushing purposes for instance), that is a breakpoint that allows you to topple neighbours quickly, that a vanilla civ usually wouldnt be able to (or with a signifcant reduction in both real and opportunity costs).
The breakpoint can also be peaceful, like Russia's Dance of the Aurora+Lavra spam, which lets you get an obscene amount of faith and production front loaded, setting up a strong snowball, much faster than any other civ can.
10% early on on the other hand, is one turn shaved off every 10 turns of production, or 18 turns spent on a civic instead of 20 turns. Or 8,8 gold per turn instead of 8 gold per turn.
Its good, but it doesnt allow for anything extraordinary that a vanilla civ usually wouldnt be able to achieve either.
Most importantly, it doesnt give an immediate edge that can be exploited heavily by the player, which is imo a good criteria for what "OP" would entail.
It does start to become quite valuable in the later game, but at that point you should already be set up, and thus it becomes a "win slightly harder" ability, instead of a game changer.
Which is why I argue that a civ like for instance Theodora's Byzantium is properly OP.
It has one of the absolutely strongest early game synergies from the Taxis ability (shared by Byzantium), and the early culture you can get from the holy sites is absolutely ludicrous.
It allows you to get an ultra fast political philosophy and +100% holy site adjacency going, and catapult you far beyond what even the deity AI can put up with culture wise, right from the start of the game.
The exploit potential for a civ like this is ludicrous, and thus firmly defines it as an OP civ.