What Mac are you using? Need specs of hardware (RAM, CPU, GPU) as this will affect solution.
If your Mac is using on chip graphics, ie does not have discret GPU, you will more than likely not get decent graphics performance in Windows. In MacOS graphics drivers are optimised for the hardware used by Apple whether discreet or on chip but in Windows drivers for on chip graphics are not as optimised, take a look at the hardware requirements for ArchiCAD on PC and it recommends against use on on chip graphics
Scott
