As I have some time sitting at the airport, here are the promissed screenshots:
Incident Creation Window before a CI is selected:
And now when I select a CI the Filter is populated immediatly:
I can scroll down now to see all the CI realted Incidents.
Hope that helps a little bit and explains, why I don't get it, that the Filter in your case needs 3-5 seconds for population.
Best regards
Fabian