IRR calculates the internal rate of return using an iterative approximation, hence it might involve quite a lot of calculations / steps.
The Excel function allows for a 'guess' parameter that might help to limit the number of iterations needed. There is no such parameter in QV.
Not sure if your data model can be improved, but a simple star model with a single (concatenated) fact table looks quite good.
Maybe the only possible improvement is to prevent QV from doing this calculations when it is not needed, e.g. using calculation conditions in your chart. Or somehow limit the scope for the calculations.