I started running into performance issues somewhere between 10k and 100k entities. I think the performance problem I encountered might have been made worse by having relationships and calculated fields associated with it. So, from what I can tell, it might be a combination of the number of entities and relations that might start causing problems, but I haven’t seen a specific number mentioned. Fibery says they have 10-20k of one type IIRC.
See discussion in Usability and Performance Issues with Zendesk Integration
Once I stopped syncing the Users type from zendesk, I stopped having performance issues. I believe the integrated types like this utilize auto-relations, which is what I suspect was causing problems between a largish Zendesk Users Type and largish Zendesk Tickets Type.