I too have the same issues! It looks like it is a common problem right now with this device. I am hoping for a fix for it in the next security patch (a man can dream can he not?). C'mon Nokia, make me be proud to own one of your devices again!
I'll give you a real life an example that happened at my work a few days back:
Colleague : "Ooooh, it is a beautiful phone! What brand is it?"
Me: "Oh, it's a Nokia 7 plus. Got it on sale yesterday."
Colleague: "Really? Didn't know they were still making phones".
Me: "Yeah they're making a comeback."
Colleague: "Can I play with it?"
Colleague: *Fiddles around with it for a minute* "Hmmm, it's a bit laggy at parts...".
Colleague: "So why did you get it?"
Me: "Well it's on AndroidOne you see. It's supposed to be fast and smooth and get updates fast... But I, I don't know why it's like this."
Colleague: "Oh..." Walks away awkwardly.
I don't think my colleague got a good impression of your phone, and to be honest, I wasn't compelled to defend the lag, mainly because, well, I simply don't know for a fact if you will/can fix this issue.
If you guys can fix this UI lag in a couple of months, I will go back to my colleague and say: "Look at how smooth it is now! They fixed it pretty quickly! You should get a Nokia! They actually fix things fast and listen." I will do nothing but praise your brand (rightfully so) to everyone who ask about it.
But for now, I'm keeping my head down.