A data processor is disclosed in which groups of execution threads can execute a set of instructions in lockstep, and in which a plurality of execution lanes can perform processing operations for the execution threads. Two or more execution threads of a thread group are issued to the same execution lane for execution. The two or more execution threads can then be processed by the execution lane successively, such that the execution lane performs the same processing operation successively. This can have the effect of reducing signal transitions, such that the overall energy consumption of the data processor can be reduced.