At www.apple-history.com I found

The IIsi came in a streamlined case used only for this model. It was originally designed for a 25 MHz 68030 processor, but shipped at 20 MHz, so it wouldn't compete with the IIci.

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Do you think I can add a 25 MHz 68030 processor to speed it up ?

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Nope, it is soldered to the board(bottom left, smallish square chip):



See the four silver rectangular canisters near the top, under the large black RBV chip? IIRC, the third from the left is the cpu clock, but I'd have to double check. It is 40.000 Mhz so to make the cpu run at 25 Mhz you need to replace this with a 50.000 Mhz crystal. I'd socket it so you can try different crystals.