Delete all iPhone bookmarks quickly

The easiest way is to go to Settings -> then search for iCloud. Disable syncing of Safari. You will be asked what do you want to do with the synced data on your device. Simply select to delete it. Afterwards, there will be no bookmarks via your Safari browser app on the device.

Is Safari opening too slowly because of all your bookmarks?

If it is, change the folder that your iPhone shows first to speed up your phone.

Choose a folder that has a smaller number of bookmarks to show up first by default.