Greg; [large data library performance]
Security checks are performed on every active ( undeleted ) dataset within a data library as it's contents are rendered upon opening. For the current user, every dataset is checked to determine if the user can access it, and if so, then checks are made to see if the user has permission to perform operations on the dataset that fall into the add / modify / manage permissions areas. These checks incur db hits for each dataset, so if your data library include many datasets ( several hundred or more ), then it will take a bit of time to render it upon opening. The size of the dataset files is not an issue here, but the number of datasets within the data library.
We are running into this limit quite a bit in practice as our data libraries grow. Splitting it does provide as a quick workaround. What would you think about loading folder data on-demand via Ajax? Our data is stored in folders and sub-folders within the library, so this would let us scale up library items without having to arbitrarily split the data libraries. I took a quick look at the code with this in might and it looked, well, hard. But that could be totally due to my ignorance of the implementation, what do you think? Brad