Can we go back to macOS 10.12 I hate 10.13

It seems that High Serra has grossly slowed down this old, tired, exposed to hurricane Harvey flood moisture, late 2013 iMac. Further for some apparently ambiguous reason Apple decided to remove the Network Diagnostics feature that I am frequently forced to used due to the amazing unreliable service of Comcast.

I chatted with Apple who confirmed that Apple deleted this feature. Unfortunately when I ask her how I could revert to 10.12.* she said that there was no approved way to do this.

Can someone verify that this is indeed the case and/or provide me with a way to revert?

Thank you,

Frustrated in South Texas (the old sub sailor)