Our University uses Sitecore as our Web CMS and we have a "dataset" component that is used to read data from lists and libraries and render it on pages in Sitecore. This was created probably 10 years ago for SharePoint 2010. We've since moved from SP2010 to SP Online, migrating (with Sharegate) the lists and libraries that needed to be rendered through Sitecore. It's been working OK for over a year (at least, folks haven't been complaining about it), but recently we found that a couple of lists are taking more time than expected to refresh. There are two in particular. One is fairly large, with lots of text data, so the ~47 seconds it takes for initial load is almost acceptable. The other is fairly small: the list has a total of 21 rows/items and about 2 dozen columns (mostly text, but one lookup with three "additional" columns coming with it). The odd thing about this one is that it was created as a duplicate of another list (using the "from an existing list" option when creating a list) and the original list is loading without issue.
Now, I'm not the creator of this dataset component, and I'm not a developer, so I have no idea how it's constructed. However, knowing that it was developed for SharePoint 2010, it's almost certainly using legacy api calls to get the data. As an aside, we bypassed SP2013 and went straight to SPO, so the notion of rebuilding this to use the REST api never occurred to the developers. They found that they could just add a new "server" for SPO and it worked.
Anyway, are any other folks still using other apps to interface with SPO data via asmx services? If so, are you also seeing slow/delayed responses from them? I know this is kind of a long shot, but worth the ask.