This ticket is detailed on https://osgeo-org.atlassian.net/projects/GEOT/issues/GEOT-6407
Steps to re-produce:
Create a working WMS Store and add layers
Stop Geoserver, go to data directory and find wmsstore.xml file, modify the URL to a valid un-responsive url (e.g http://localhost:1234/geoserver/ows?service=wms&version=1.1.1&request=GetCapabilities)
Fire up Geoserver
Startup sequence will show WMSStore trying to fetch remote styles, and taking substantial time to fail increasing Geoserver startup time.