Well - related to this, I found a TIN surface of the site I want to section, is too big to Copy & Paste, and it also fails to work as a Layer Export / Import.
So, I got the TIN surface done - and DataCAD gives me a warning that it could take 18 - 19 minutes - and it is done in barely longer than 3 minutes, on my 8 core Ryzen computer.
You could try turning Background off in the Section Tools so you only get the cut line profile without the HLR mesh beyond? Also check you z/b-z/h settings are correct for the section cut.
Thanks - I got it to work, finally. There was a lot ... stuff ... going on. When I exploded the symbol that had the topo lines - it was flattened. I tried copying and pasting the topo lines, but there was a very strange enlargement thing going on. I was finally able to make a symbol of the TIN model I had made in the symbol.