Worlds and systems with a lot of compendium data will take longer to start up on v12 compared to v11 due to a more secure world migration framework in Foundry, but this applies to the server startup, not after you reach the Forge’s loading bar.
When the world loading step (with the loading bar) takes a very long time but eventually succeeds, it’s usually due to one of three things:
Connection speed
World size
Amount of processing happening in the browser (modules start up at this point)
I’d recommend testing to see if the slow loading persists if you have all modules disabled, since that would be a good indication that a module’s working in the background may be contributing to the slower loading time.
The issue with receiving a message that you weren’t logged in seems to indicate that your browser cache/session cookie might’ve expired and need to be cleared. Extensions can also play a role here so if you consistently run into slow loading speeds, check to see if it persists in incognito mode or on a different browser.