Probably reasons: maybe background image is too big? or big count of bookmarks in subfolders? https://bugzilla.mozilla.org/show_bug.cgi?id=1396364 Option "Remember last dial group" can also slowing browser by additional browser.bookmarks.get (it working slower than browser.bookmarks.search).
Another reasons may be in some of installed extensions: i have seen same thing with "Tab Session Manager" and option"Save the session when window was closed" some times ago; maybe some another extension can cause this: about:debugging.