I've always found Google's decision to include mid-tier SoCs into their flagship phones risky as it makes performance hitches for future updates much more obvious. If/when Google copies Liquid Glass into the next version of Material Design, I'm sure my phone will suffer from a performance hit too.
That said, scrolling HN still works fine on hardware from a decade ago, so there's got to be more to this. I've personally had custom ROMs experience random lags and slowdowns after major upgrades (which is probably why many ROMs claim it doesn't work and don't support it) and I wouldn't be surprised if the Android upgrade hit a similar issue on your phone. As a last resort before buying a new phone, doing a factory reset may make the new OS more usable on your device. Not the right solution (fixes from Google's side to prevent such issues would be right solution), but it might work and it's cheaper than a new phone.