When loading the Admin UI Events tab, the filter data is also loaded. These takes some time (8+ sec) on systems with many series defined. Same bad performance happen requesting the event metadata.json.
On both screenshots you can see the request to the filter.json and the metadata.json need 8+ seconds for a response. The testsystem has 1000 series defined.
Internally the SeriesListProvider is called for a list of all series in the system. The call take the most of time.
I've added a bash script to attachments. It will create 1000 series for testing. Here you can find a script to create a large series range for testing in more comfortable way.