BCS on office 365 doesn't perform very well, and needs to be improved
We have found on a project where we have used BCS to connect to on-premises databases through OData web services.
After a support call these are the Microsoft support team conclusions:
SharePoint On-Premises environments are dedicated environments where as SharePoint Online is a shared environment, hence it would be slow as compared to SharePoint On-Premise.
Since the SharePoint Online List has to fetch data from the External Database, it would take some time for it to fetch the data & save the changes to the External Database.
This uservoice idea is asking Microsoft to review the BCS functionality and improving the performance.