This is what I think is happening. A single formula that both sorts and filters the Drawing_Vault list results in a query that causes SharePoint to lock the underlying SQL Server table. This is the cause of the "Exceeds Threshold Enforced by the Administrator" error. Separating the filter and sort operation into 2 different places in your app simplifies the query and prevents this error from happening. The post here contains a bit more detail on this behaviour.