Hi Sunnie,
There are two types of performance indexes in Territory Design.
(1) The Territory Index - this is an adjacency index that essentially builds a lookup table for each geography so that the software does not have to do a complex spatial operation when assigning or swapping territories. It is such where each element, for example a ZIP Code knows all of it's adjacent neighbors by ID - this makes the territory building fast and efficient. Territory Indexes are created on-the-fly when you build a TD solution. All Territory Indexes are saved to the location where the alignment layer is housed as a .TIDX file. If you've installed the Business Analyst data you'll see these for all standard datasets already created here: C:\ArcGIS\Business Analyst\US_2017\Data\Demographic Data\USA_ESRI_2017.gdb. You can delete these TIDX for any layer and they will be recreated the next time you build territories with them.
(2) Territory Network Index - this is a performance index that acts like an OD-Matrix. Instead of the software having to calculate every single street feature, it uses a higher set of points to mimic the network. If say using Los Angeles as an example, you could use a subset of say Block Group centroids as the Territory Network Index points. Then when you select the network index in the territory connection, it should process much much quicker.
Hopefully this is helpful and this should make your analysis successful. Nationwide territories are perfectly normal, but using a street network for distance is an extremely intensive process. Try creating a Territory Network Index to help with this.
Kyle
