Like many, as I've starting ramping up the clock speed on my 6502-based SBC, it's the limited speed of the EEPROM (the AT28C256) which is the main limiting factor. A great replacement is to switch over to the DS1230Y Nonvolatile SRAM chip, which is pin compatible with the AT28C256 but much, much faster.

Using the TL866II+ programmer, you can upload your "ROM" image quite easily as well:

minipro -p "DS1230Y(RW)" -w ./minios.rom