This is more of a curosity question than anything else, but if one uses the undocumented instructions to access the lower and upper bytes of IX and IY, does it take -- approximately -- twice the number of T-States as it takes for their equivalents in H and L to execute?

This is more of a curosity question than anything else, but if one uses the undocumented instructions to access the lower and upper bytes of IX and IY, does it take -- approximately -- twice the number of T-States as it takes for their equivalents in H and L to execute?

It adds 4 clock cycles (which are used to execute the $DD or $FD prefix). Note that you cannot load these upper/lower bytes of IX/IY to/from memory though.