Safari beach-balling on some websites like Github.com Safari has been stalling when I load some websites. Github.com is the most reproducible. The page would load, but then my mouse would turn to a beach-ball and just get stuck until I close the tab with Github or whatever site seems to be causing it.

I am not running any extensions - to be honest I don't know what it could be. My boss is having the same issue as well. I have no issues with loading Github on Chrome.

I am running macOS Sierra (10.12.1) on a Late 2014 Mac mini and Safari 10.0.1. It also happens on the Safari Technology Preview (Release 26, 10.2)

What are ways to fix this or further diagnose slow web sites in safari?
Release 26 of Safari TP contains many changes to the rendering engine. This causes problems with many websites, especially where image sprites are used extensively.

This will likely be fixed in the next release, but until then, you can just use the standard install of Safari, or downgrade to the previous version of Safari TP if you still have the installer. You can also try running with Disable Caches selected in the Develop menu.