"Half the tech rate" is not really a hyperbole.
I just checked with Babylon: Starting the 3000 BCE scenario, normal speed, these are the initial research durations for Mining, owning just one city and having the slider on 100% research:
Heir: 8 turns
Regent (standard difficulty for DoC): 10 turns
Monarch: 11 turns
Imperator: 13 turns
Paragon: 14 turns
It's just the first tech, and already 40% longer research duration. On higher difficulty, maintenance is higher, too, so the tech slider is going down even sooner.
Not a problem with Babylon (unless you want to reach their tech-based UHVs), but a severe problem with larger empires.
I experienced that recently with China (1.16 develop): You need to expand that civ, and fast, cover your resources with plantations for happiness, and then grow and whip your cities, churn out wonders, soldiers and temples. For all that, you need techs, but with the respective longer research times, you have quickly built all available buildings in your first cities... so, go for the building-techs? Build another dozen workers and pave every tile in your proto-empire with roads? Produce archers to immediately destroy them, so you don't have to pay unit maintenance? Definitely not build settlers, because more cities will bog you down even more. Well, now I'm talking strategy here, and on higher difficulty, you need to completely rethink that strategy.
And what Dan mentioned, is specifically the tech UHVs: While your tech rate goes down and your maintenance up, the AI has just the reverse going for them, allowing them higher teching sliders. Without exploiting (and yes, I'd call some of my civ switching exploits) the UHVs are beyond "challenge".
@Leoreth, I don't blame you on that, it is my decision whenever I'm not playing on Regent; and balancing 50 civilizations on every difficulty and game speed level doesn't need to be done by you, yourself. You developed the game for Regent/Normal, I assume, and that's fair enough.
If there are more guys like
@DanLT3 and me, we could playtest various difficulty levels and speeds. (I could even try to not civ-switch for that, *gasp*)