It is odd that it is slow. On my 2019 macbook pro

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new more realistic example: time vim -c 'syn off' <64 MB>.txt vim -c 'syn off' <64 MB>.txt 0.41s user 0.20s system 32% cpu 1.848 total

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Here is my first, pre edit, example which is invalid. The file was a zip and my install of vim was not opening as text or binary

% time vim -c 'syn off' <48 GB file> vim -c 'syn off' <48 GB file> 0.03s user 0.03s system 2% cpu 2.380 total