I’ve had similar problems with my new mac and I would like to affirm that it isn’t your source material and it isn’t your plug ins. I work with SD footage and my new mac doesn’t have the hardware to show a clip with a Gaussian blur on it, but the old Mac can process it just fine. Just one example of the many problems I’ve had since receiving my new mac.
It’s aggravating that the same material works flawlessly on older machines, but shudders and craps out on a shiny new Mac.
Sorry I have no answers and can only empathize. We’ve talked to the arrogant and asinine FCP “support” with no resolution. Apple screwed up and they don’t know how and they won’t admit it.