[jonsen]

A shift is faster than an add.

[russdill]

On 6502 they are both 2 cycles. On the vast majority of processors I'm aware of, "simple" alu operarations don't vary in execution time

[kragen]

On 8086 or ARM you can get a shift "for free" with your add, although on the 8086 you have to use the LEA instruction.

[ksherlock]

ADC #immediate is 2 cycles, as is ASL. Are you really multiplying a constant by 3 at runtime? Probably not. ADC zp is 3 cycles. Plus 2 cycles for the CLC.

[ForOldHack]

The M68020 added a dedicated barrel shifter, which could execute much faster then the M68000.