I’ve been experiencing a spike in CPU utilization on my 2019 i7 Macbook pro running Mojave and have been able to pin it down to one specific website - xero.com When logged in, regardless of whether shields are up or down, it sits at over 100 CPU utilization. It does not seem to be an issue if you are not logged in, but only if you are a logged in, authenticated user of the website.
Issue present whether default shield settings are up or if they are turned off.
Site does not cause CPU spike when using Chrome (checked with both logged in and non logged in user).
Version 1.20.103 Chromium: 88.0.4324.152 (Official Build) (x86_64)