Complex Instruction Set Computers (CISC) improve performance by reducing the number of instructions per program.
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For CISC, many complex instructions with variable length means: ? Control units need to use special circuits for interpreting instructions as they are fetched from memory, this can take additional time. Instructions may take multiple / variable numbers of cycles to process hence harder to manage pipelines. Easier to program, fewer instructions required for complex operations.
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For CISC, many instructions can access memory using many addressing modes which means: ? Parameter passing through memory means additional strain on von Neumann bottleneck. Fewer registers needed, fewer operands per instruction.