I believe that was part of the improvements to loading screen times. Rather than loading everything behind the loading screen, it now drops the loading screen and loads the last textures while you're playing if the loading screen has been there too long. Sounds like in this case though it might be dropping the loading screen too early, if things like staircases are still missing.
Almost every time when I'm going to master crafter merchant in Mornhold I can see the second loading screen. I'm appearing near wayshrine, running to the merchant, opening his store interface and then there is a short loading screen (3-5 secs). After that the store is closed and I'm just standing near the merchant as if I didn't talk to him.
Also I see low-resolution textures, black player models, can't see the most part of environment sometimes right after arriving in capitals and other places with a lot of players.
I think that it is definitely depends on HDD speed. The game can't load all the data as quick as needed (during the loading screen), so this data is loaded asynchronously when the player is already in the game.
Also every time when the game is stuttering, I see that my HDD usage percent is going up. I suspect that it's because 8GB RAM is not enough or my HDD speed is low. Seems like the game is using Windows page file in these moments and it leads to stuttering.
By the way, tomorrow I will do upgrade. I'll add another 8GB RAM and SSD. So, I'll check how the game runs with 16 GB RAM first, and then I'll install it on SSD and check it again.
UPDATE: nope, seems like 8 GB RAM is pretty enough. I've checked disk usage in Windows System monitor. The game doesn't read pagefile.sys at all. The most disk usage is made by reading many of *.dat-files in /depot folder of game folder. So, I think that all that stuttering and additional loading screens are because of the low reading speed of game data. Maybe SSD could help with that.
Well, I've added 8 GB RAM (so now I've got 16 GB) and SSD (Windows is installed at SSD, the game is at HDD).
And now there's no stuttring at all. Two days of testing and I didn't notice any stuttering. Also no low-res textures anymore.
Some items and NPC models are still invisible for 15-30 secs right after me arriving in the new zone though. And there was a second loading screen two times. First it was in Wayrest when I was riding a horse fast from the wayshrine to another side of the city. And second was in Mornhold, and it was a neverending loading screen bug.