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I also use Altium to design big PCB like you. But it doesn't have slow problem.I think it may be your computer problem.
Maybe your computer and the PCB Editor setup influence your computer speed.
1. Some virus checkers check every file access, which slows the operation of graphics intensive CAD programs.
2. Turn off online checking in Altium Designer.
3. Clean your computer with enough power for its operation.
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