$ time vim -c 'syn off' tt.json
real 0m3.277s user 0m1.690s sys 0m0.349s
So IMO I think vim is being "too dumb" here and should be able to adapt like VSCode does. But, meanwhile, if you want to test under equal conditions, you can disable VSCode's optimization by disabling this setting:
Editor: Large File Optimizations
Or directly in settings.json:
"editor.largeFileOptimizations": falseDisabling the advantages of one application vs another is just kneecapping the superior editor IMO.
edit
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