Thank you for the profile! It looks like a duplicate of Bug 1818461, the problem is not just Sync or Places doing a large operation, but it's things observing Places doing synchronous work inside the notifications. There's absolutely no reason to update the bookmarks toolbar empty message this often, and moreover using synchronous IO during the notification itself (that is where most of the time is spent here, unpredictable I/O probably causing some thread contention). That's what Bug 1818461 is tracking, we can make that a ton more efficiently. There's another offender in the New Tab page too, I am tracking those as blocking bug 1533061.
Bug 1813545 Comment 15 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Thank you for the profile! It looks like a duplicate of Bug 1818461, the problem is not just Sync or Places doing a large operation, but it's things observing Places doing synchronous work inside the notifications. There's absolutely no reason to update the bookmarks toolbar empty message this often, and moreover using synchronous IO during the notification itself (that is where most of the time is spent here, unpredictable I/O probably causing some thread contention). That's what Bug 1818461 is tracking, we can make that a ton more efficient. There's another offender in the New Tab page too, I am tracking those as blocking bug 1533061.