@readingdancer While I appreciate you testing this theory out, the Dashlane extension causing a slowdown is a verified issue that can be seen easily by disabling the extension on the static preview of the backoffice at https://backofficepreview.umbraco.com, for instance. Other extensions, such as LastPass, do not cause similar effects.
If you would like to share your findings on the issue you created, that would be greatly appreciated.
Both issues can be real at the same time.
I agree about Dashlane with Umbraco, I was just letting you know that I tested with and without Dashlane running, both of my tests had similar timings, so for my tests it didn’t seem to make much difference, maybe they have done something to help fix it, or maybe I was just lucky 
It seems like Dashlane is no longer a problem. I just tried reenabling it, and not noticing any degradations yet!
Is this for 17.1 that you tested?
We released key optimizations in that version, and it seems we have gotten below a certain threshold of which, if you were over, the browser “went crazy”. Not sure exactly what fixed it, but anyway, we are continuing the journey for 17.2, which should be even faster.
I tested both in 17.1 and 17.0.2