Thanks for your questions and thoughts @chun_hing_yap and @Syed_Muhammad_Qadeer!
Is your question about Central or about Collect/clients? The biggest performance issues we're aware of are on Collect both related to data transfer and display/filtering. This functionality is aimed at helping with Collect performance by reducing the number of Entities individual devices get. Are you getting performance issues on Central?
Would that be half a million to a single device or would you be able to divide up the list with functionality like what's described in this thread?
Entities are served to Collect as attached CSVs and Entity Lists are optimized in Collect so I'm surprised this is your solution. Is it because when you use a manually-built CSV/JSON you can change the subset of items attached? For example, the first 100,000 on week 1, then the next 100,000 on week 2, etc?
How many individuals need to access the same group of Entities in your scenario? Could that group share an App User token as a stopgap before we introduce a formal group feature?