I have similar issue. I have very heavy projects with tonns of plugins, tracks, busses etc… created in CB 11 Pro and they works fine without issues there but when I open them in CB 12 Pro even with same ASIO Guard High - project start stutter, I have dropouts and just overall CB12 Pro feels struggling and a lot slower in heavy projects. It seems there need to be done some improvements in CB12 engine or improve performance to state like CB 11. Otherwise CB12 is useless for me now - it feels laggy and unresponsive in huge projects for now. Will stay on CB 11 Pro for now until Steinberg fix and improve performance to same level as CB 11 Pro.