Very slow access of activity screen

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) - batmat 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. 

Support bundles: 

https://issues.jenkins-ci.org/browse/JENKINS-44995?focusedCommentId=320047&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-320047