In the Task Manager, if you right-click the ArchiCAD.exe file you can use the Set Priority command to change its priority.
According to Windows articles on the web, increasing this Priority will give more resources to the given process (which might include more RAM).
Actually, I have never tried it myself so please use it as your own risk. There are different opinions whether setting it to the highest value (Real-time) will slow the whole system down or not. In any case you can set it to 1 or 2 notches higher and see if that makes any difference.
There is another thing I just thought of. On my machine I set the paging file (virtual memory) to a relatively low value, like 1 GB. This way the machine cannot swap your data from memory to the virtual memory and will keep more data in physical memory. The reason I did not set it to zero is because I don't know, maybe it needs some amount.
So this is another thing you can try.
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Loving Archicad since 1995