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The combinational critical path is a longest propagation-delay path through a combinational logic circuit. Register-to-register clock timing also includes launch delay, setup time and clock uncertainty. Putting two functional units in series adds their dependent delays; operating them in parallel instead contributes the slower branch plus any subsequent selection delay.

Instruction pipelining divides execution into stages with stored intermediate state, allowing different instructions to occupy different stages simultaneously. The processor clock period is constrained by stage timing; moving a barrel shifter and arithmetic logic unit into the same stage can lengthen that stage's combinational critical path.

The likely effect in a single-cycle or unchanged pipeline-stage implementation is a longer combinational critical path and hence a lower maximum processor clock frequency. Previously the barrel shifter and ALU could work in parallel before result selection; the modified path can require both shift and addition before address or result use.

Ignoring common processor register and wiring delays, the relevant combinational logic contribution changes roughly from $\max (t_{\mathrm{ALU}},t_{\mathrm{shift}})+t_{\mathrm{mux}}$ to $t_{\mathrm{shift}}+t_{\mathrm{ALU}}$, with any additional input selection included as needed. The processor clock period must also cover processor register timing and the rest of the instruction path, potentially including memory access. Removing a multiplexer offsets some delay, so a numerical frequency change cannot be inferred without component timings. Pipelining, a faster implementation or another pre-existing bottleneck can alter the outcome; the changed serial dependence is the reason for the likely reduction.

The reciprocal of the processor clock period. Reducing a combinational critical path can permit a higher frequency, subject to the remaining timing requirements.