There might be a lack of statistics gathering tools that would help isolate performance issues. 5.3 will have the Filter+ features for timing the View refreshes. That’s a start.
Given that some users with larger trees than the 100k you reference are not seeing the same performance issues with SQLite, more has to be done to gather information.
For instance, I can see that large Note content editing is VERY slow. A Paste can take minutes.
On Windows, I had a Place hierarchy that made the Grouped Place Selector grow slower as a session lengthened. (Exporting/re-importing and Repair made no impact.) That slowing did not happen once I hacked the selector to be a Flat list instead of hierarchical. (Thanks again Dave!) Nor does it happen with the same tree on Fedora.
Without isolating the causes, those slowdowns are unlikely to be eliminated.