Emulation Speed
By using the tools menu "Set Cpu Multiplier" you can change the speed of the emulator.
I recommend you do not use values greater than 3 (12Mhz).
You can put real numbers as well i.e. 4.3
Newbrain real speed should be around 0.6 of the 4MHz since on a real Newbrain the cpu does nothing when the screen is displayed.