The CPU evidently is a 6502 ("R65C02P4") and you should be glad that it runs at 6 MHz. Most of them run just at 5 MHz. Usually this CPU can't get tuned.

If you want to tune the board, the first item should be changing the CPU. Try a modern WDC CPU that might run at 10 MHz or higher. You can get it from Mouser's, USA or Mouser, Munich, Germany.

Walter