A data processing system is provided for setting a value of a performance controlling parameter during processing of a data stream comprising a plurality of data blocks. The performance controlling parameter is set by deriving a complexity measure for at least one data block by performing an initial processing stage on the at least one data block. The performance controlling parameter is set to a predicted value in dependence upon the complexity measure and at least one further processing stage is performed on the at least one data block at the predicted value of the performance controlling parameter.