I wanted to chime in and let everyone know we are having a similar issue with our Jenkins setup using the Pipeline plug-in. Each page load in Jenkins was approaching 11s, which is unacceptable for our CI pipeline and causing friction on our team.
I found that this was the issue by sampling the Java stack every second during a GET request, here are the results in a gist. I saw the common denominator of each sample was the BuildUtil.getDownstreamBuild, and found this ticket.
We have 7 pipelines in our installation, each having about 6 jobs, and most of those jobs having 1-3K builds in their histories. I purged the histories of these pipelines, reducing their histories to about 300 each (total 60K -> 17K), and Jenkins pageloads are now 2s. I had to do a Jenkins restart after the purges.
We are running Build Pipeline Plugin 1.5.4 and Jenkins 2.30.