beachballing

Activity Monitor will reveal an application or process that's "not responding" in red. You can force it to quit, if necessary.

If it is Safari, then the following Apple Support documents are relevant:

• Safari is slow, stops responding, quits unexpectedly, or has other issues - Apple Support
• If Safari doesn't load a page or webpage items are missing - Apple Support
• How to use Internet plug-ins in Safari for Mac - Apple Support
• "Safe Mode": Use safe mode to isolate issues with your Mac - Apple Support. Determine if the problems exist in Safe Mode. Then, restart normally.

With Activity Monitor open and the CPU section selected launch your browser and go to the websites that are giving you problems. Watch the CPU usage to see what is using up the CPU resources. Take a screenshot of it and post with your reply.

How to use Activity Monitor on your Mac - Apple Support

Well just watch the Activity Monitor and when you get the beachballing see what process is eating up all of the CPU resources.