Have you experienced the Safari SBBOD (Spinning Beach Ball Of Death)? I am having that problem in macOS 10.13.3, so is my wife. Don’t know what causes it, but if you are suffering this painful behavior in Safari, I have good news! I am currently testing macOS 10.13.4 Beta 6 and this behavior is gone. The 10.13.4 update should be released very soon, stay tuned.