@Maorui2k From your description the issue could still be caused by some of your machines instead of the switch being the bottleneck. Again, I am not saying that switch isn’t causing this. Just want to point out that testing PCs in batches doesn’t cost you money (like a new switch would) but only some time.
Multicast in FOG works pretty similar to how Ghost does it. Although back at my old working place we had strange performance issues which we never had using FOG multicasting!