Some users see activity screen as very slow when there are a large number of runs (this seems to be the more common and serious case...)
The best example of this is https://ci.jenkins.io/blue/organizations/jenkins/Core%2Fjenkins/activity - note how long it takes to load (compared to other pipelines there). You also see this if you open from a PR and then dismiss the result screen (as it goes back to activity) - Baptiste Mathus reported this.
(this seems to be slow even when anon, don't need to be logged in, but may be related to favourites too when logged in)
— NOTE ----
This ticket was split from https://issues.jenkins-ci.org/browse/JENKINS-44995
so please see the support bundles/info there as it may be relevant.