Can the community here give some precise information on why the AoC client performs a lot better than the HD one?

See: Title.

I've been talking with a good friend of mine about why the Age of Empires II HD client can often feel like a kludgy mushy mess, but they're not convinced by me simply saying "Because it just is", and not happy with me explaining that the original HD devs made a mess of the codebase, and then the Forgotten Empires team were made to work from that codebase, rather than starting the HD project over again from the original AoC codebase.

They wish for proof, and explanation, for why the HD client runs mushily, or feels like a mess.
They claim to me that they can't see the difference, and that people are just whinging for the sake of it, but I know from my own personal experience that there is a big difference between the two clients.

Would it be okay if I asked the community to help give some history, and some explanation, as to both what is wrong with the HD client, and why/how it ended up at the state that it is now?

(Also, is the DE client being worked on from the AoC codebase, or from HD?)

EDIT: They're also asking for sources, otherwise they'll reckon y'all are just parroting on what other people have said to you.
I trust the guys here, but, well, they just wish to be sure!