99% the Library issue is due to running a query on the main-thread while the helper thread is doing heavy SQL I/O. Not a new thing unfortunately, the history/bookmarks views are still executing queries on the main thread. We can move this work to a separate connection to solve this kind of contention, it would still be on the MT but not await for async I/O. Though, I think the discussion is going off-topic, OP said the new sidebar is slower than the old sidebar, but comment 1 measured the Library. Those are different things, and while I'd be glad to get a bug on file for the Library contention (please file it!), the sidebar issue must be investigated yet.
Bug 1964023 Comment 8 Edit History
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99% the Library issue is due to running a query on the main-thread while the helper thread is doing heavy SQL I/O. Not a new thing unfortunately, the history/bookmarks views are still executing queries on the main thread. We can move this work to a separate connection to solve this kind of contention, it would still be on the MT but not await for async I/O. Though, I think the discussion is going off-topic, OP said the new sidebar is slower than the old sidebar, but comment 1 measured the Library. Those are different things (the grouping may be completely different), and while I'd be glad to get a bug on file for the Library contention (please file it!), the sidebar issue must be investigated yet.