@keropi
I think there was an upgrade for the 486slc 25mhz that was avail. it was a chip that clipped on over the original CPU. I think it might have been from Evergreen or something similar. I don't know if this would be a total answer for you but it might be an option to get a faster CPU.
Here is some info. I found not much but give it a try and see what happens
LinkHere is another
Link