Old projects and projects mad on PC are running super slow and are unusable

Hi there,

  My very kind company recently bought me a shiny new Mac Pro (2019). On most things, it runs like a dream, except some projects in After Effects.

If someone has shared a project with me which was built on a PC for instance, AE on my mac takes an age to load it, and then it constantly gives me the spinning wheel of doom. 

If I try and load an old project I made it does the same thing.

If I start a project from scratch.. it runs perfectly as you would expect.

Are there issues with using PC projects on a Mac ??

Any help greatly appreciated

Alex