The IIsi was designed as a less expensive, less expandable alternative to the Mac IIci. Cost saving measures included eliminating NuBus expansion slots, soldering 1 MB of RAM to the motherboard, and using a slower CPU (20 MHz vs. 25 MHz). Although the IIsi was marketed as a 20 MHz computer, users quickly discovered it used parts rated at 25 MHz. (Apple had intended it as a 25 MHz computer, but chose to scale back the speed to avoid cutting into IIci sales.) Chipping the IIsi to 25 MHz -- or even 28 MHz - is not unusual.

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Because of its heritage as a cut-down IIci, a simple modification carried out by many owners was to substitute a new clock crystal to increase the speed to 25 MHz.

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