The processor on that thing is factory-rated to 20MHz, it wasn't an underclocked 25MHz chip thrown in by Apple.
Anyway, that's one of the Macs where a clock crystal would need to be replaced (like most 68k and early PPC models).
I don't have a IIsi in front of me, so I can't say exactly where it would be, but there's a 20MHz oscillator somewhere on the logic board (either a small two-lead can type or, more likely, a larger rectangular type) that will need to be replaced with a faster part. There are likely a couple of different oscillators on the board, so make sure you replace the correct one.
Anyway, that's one of the Macs where a clock crystal would need to be replaced (like most 68k and early PPC models).
I don't have a IIsi in front of me, so I can't say exactly where it would be, but there's a 20MHz oscillator somewhere on the logic board (either a small two-lead can type or, more likely, a larger rectangular type) that will need to be replaced with a faster part. There are likely a couple of different oscillators on the board, so make sure you replace the correct one.