Important performance factors
Index Manager is a highly I/O and memory intensive application. The defining factor that affects its performance is where you choose to store the folder that holds the indexes and the location of the cache. Ideally they should be stored on the fastest local drive your server can offer; preferably a drive with a large cache.
Since indexes are relatively small in size, storing the indexes on an SSD would be the ideal solution, your budget allowing.
The same goes for the cache folder - because Index Manager delivers thumbnails and previews to FotoStation and FotoWeb clients, having fast access to cached assets will have a very beneficial effect on the overall performance of the system.
Tip: You can use Windows Performance counters to measure Index Manager performance on the server.