Triple-click the text on the line below to the clipboard, then copy it to the Clipboard (command-C):
Quit Safari. Disable Safari synchronization in iCloud, if applicable.
Go▹Go to Folder
from the Finder menu bar. Paste (command-V) into the text box that opens, then press return.
A folder window should open with a file named "Bookmarks.plist" selected. Move the selected file to the Desktop, leaving the folder open.
Relaunch Safari. It will open with the default set of bookmarks. Delete them all. Select
File ▹ Import Bookmarks
from the Safari menu bar. Import from the bookmarks file you moved to the Desktop. Arrange the bookmark folders as you wish.
Re-enable synchronization if you disabled it. If Safari now performs normally, you can delete the old bookmarks file. Otherwise, quit Safari again and put back the file you moved, replacing the newer one with the same name. Close the Finder window and post again.