Safari beach-balling on some websites like Github.com
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.