The /activity and /runs endpoints have been reported to take up to 28 seconds on some Jenkins masters.

Tracing through the code, it looks like io.jenkins.blueocean.service.embedded.rest.RunContainerImpl#get will load all the runs using hudson.model.Job#getBuilds() for the Job then applies the pagination.

However, there is a method hudson.model.Job#getBuilds(hudson.model.Fingerprint.RangeSet) that will allow us to fetch a range of data which would be suitable for pagination.