I'm running lightroom on a 'decent' PC (4700K, 16GB ram, ssd system drive, storage spaces for storage pool inc. photos, 220/22 Mbps internet)& lightroom seems to take ages to startup - from a few minutes to.. basically never (30 mins). What is it doing in this process? If it could be a long time why aren't there proper progress messages? And surely the UI should basically come up even if photos are greyed out. It's a frustrating UI design
Try logging out off your Creative Cloud account and then back in. Has worked for me on occasions.