I'm just not seeing it. Late game progression may be sped up a bit, admittedly. But Quill cleared the tech tree in his game at about 300 turns and he was maximizing science. He was suzerain of two science city-states with no contest, and went almost virtually uncontested with every single great scientist in the game - I believing missing one, and passing on another. In civ5 on prince/standard/standard I can max the tech tree in about 350-400 turns, maybe even less - That's not really that far off. I don't consider myself a Deity lvl player.
These playthroughs are providing skewed information due to the difficulty setting vs the player because of the sheer advantages that favor the player. Just like wonder competition is fierce on higher levels in civ5, so will things like suzerain bonuses and great people competition become meaningful due to the advantages that the A.I. has available to them in higher difficulties.
In civ5 on prince, you're guaranteed a great library sling-shot and crazy science boost because of it. Depending on your skill level, the higher up you go in difficulty, the less of a reality of doing that sort of thing exists. It's the same thing in civ6. Like I said, perhaps late game progression needs to be tweeked - but in all of the comparison games I've run to come up with some sort of analysis to prepare for these kind of debates - the tech rate is almost identical between both games when it comes to the fist 100-150 turns.
I am not surprised that Quill got nearly every scientist in his Rome game. He's playing on Prince. If he were playing on Emperor or higher, I would be concerned. The fact that he got so many scientists and suzerain bonuses, as well as wonders - all things that will be harder to do on higher difficulties, make the result of him flying through the tech tree a perfectly reasonable outcome as far as I'm concerned. This should not happen with a bad player or a higher setting.
I get what you're saying - if Quill were actually playing competently he'd do even better, and you're right. But Again, I can't see this as cause for concern because of how low the difficulty is. To some players, Prince might as well be Settler. I just don't think being able to nab all of a great person type and maintain suzerainty over the vast majority of city-states is going to be the normal state of the game. That's like suggesting that building every wonder in a city civ5 is normal - that just doesn't happen to most players that play on any reasonable difficulty.