Dito. Thanks for swift and detailed responses.
I can't say for sure. I haven't really paid any attentien to the loading time of the dashboard until it took 40 seconds. But no, I would not say it is noticeably slower.
A known issue (and link from the release note) has been added for this change. https://puppet.com/docs/puppetdb/6/known_issues.html
For the time being we plan to avoid the issue by disabling the JIT by default for all PuppetDB DB connections via PDB-5652.