It seems that Fallout 4 takes more time to load if you are playing in full screen.

Fullscreen mode: Even though the majority of the users think that using a full screen reduces the load of the computer, in fact, it is quite opposite.

Vertical Sync: NVIDIA’s vertical sync is known to improve the gameplay of several PCs but it is also known to cause several different conflicts and issues with the games which aren’t optimized to run with it.

Performing a workaround for this might fix the issue.

Bug in the game: Even though this is a rare occurrence, we found several cases where there was a bug and the game was taking longer than usual.

Overriding this setting in the configurations file might fix the problem.

Threads: Another instance can be where there are not an appropriate number of threads being run/executed for your game.

Slow disk access means that the game will take more time transferring data from your HDD drive to your RAM.

Slow disk access: A lot of our users reported that it was because of slow disk access that they were having issues when loading the game.

Fallout 4 textures take forever to load