I never thought I'd say I missed anything about SharePoint 2010. The "Datasheet mode" in SP2010, while limited to Internet Explorer and sometimes complicated to troubleshoot (because it used Access 2010 components) was reasonably reliable and generally pretty quick at importing many rows of data to a list.
The initial implementation of Quick Edit mode in the Modern experience was terrible. I mean just awful. Trying to paste in more than about 15-20 rows would take forever and often items were duplicated. Whatever was causing that duplication and the severe lag was finally addressed and it worked OK. Still not as fast as Datasheet mode in SP2010, but acceptable (esp. considering we aren't tied to IE any more).
The latest iteration/update (which supports JSON column formatting and adds some other bells and whistles) looks great. BUT, it's agonizingly slow. Pasting in 4 columns of data (all "single line of text" columns) takes 2-3 seconds per row. I have folks who routinely have to paste in 200-300 rows (with 10-12 columns) of data. Those are taking 5-6 seconds per row.
I realize there are other means of getting data into SharePoint lists (e.g.: Power Automate or third-party apps), but something called "Quick Edit" should (at least) be quick. Maybe with the introduction of Microsoft Lists, some attention will be paid to this. I know creating a new list from a spreadsheet works pretty well, but there needs to be a better way of getting data into existing SharePoint lists. In many cases, those lists are tied to processes or apps, so we don't want users to keep creating new lists just to get more of the same type of data into SharePoint.