Hi!
I suggest you check your computer performance using Puget Systems Benchmark Tool for Pix4D. Please share your results with us once you finish testing. Please also remember that the current version for macOS 3.0 is no longer supported. Learn more Beta macOS version of Pix4Dmapper 3.0
Trying to get a more realistic 3D image please have a look at our two articles:
How to improve the Outputs of Pix4D Desktop
How to improve the 3D textured mesh
To generate the Contour lines you can do it from the bar Process >> Generate Conture Lines without re-running Step 3 if the DSM was already generated.
Cheers!