We don’t know the reasons why GP becomes slow on some macOS systems. We have seen a couple of reports about lags on M1, but it is not clear if they are actually caused by M1 or by something else (e.g. screen): Gantt software started lagging while using Apple M1
We also tested GanttProject on M1 laptops which we could find and it worked fine.
One possible workaround which might help is to install a Java Runtime for M1 chip and test if GanttProject ZIP distribution works fine with it. I suggested doing this at the end of this topic: Gantt software started lagging while using Apple M1 - #15 by DmitryBarashev but I didn’t receive any response from the topic author, so I can’t claim that it fixes the issue,