Practical limit to number of workitems assigned to a single iteration?

I am working with a customer that is doing large projects with Earned Value Management (EVM).  Since they have fairly long iterations (2-3 months) and have to keep their tasks small for EVM, they end up with a large number of tasks per iteration.  They currently have an iteration with around 600+ workitems, and they are experiencing performance issues when adding a predecessor or changing a priority in the Iteration plan (takes about 5-15 sec per transaction).

What does IBM recommend as the max number of work items within a given plan?