The CPU divider is from the address bus (in most cases, the CPU program counter). I assume it's due to slow EPROMs, similar how for example Mephisto Glasgow has wait states with DTACK.

No it's not a switch, it's a wire. The only way a user could change it is opening the chesscomputer and resoldering the wire or removing it.
(and maybe break the chesscomputer when the EPROMs can't handle the increased speed... so don't try it )