But I'd like to convince you and everyone that in general we want to accept inputs in any form and be form-insensitive because that's much more user-friendly.

We don't in fact have universal NFC-only input modes. We have accidentally NFC-mostly input modes -- accidental because typically we have transcoding from legacy codesets, which yields NFC, but no normalization is actually done so that copy-paste operations can cause non-canonical input to be provided to applications. We don't have universal agreement on NFC. This makes for a mess with occasional user-visible problems.

Form-insensitivity is essentially the same as normalizing character-by-character when hashing or comparing strings, but this can be very fast for mostly-ASCII text, so form-insensitivity can be very fast.

For string comparison form-insensitivity is faster than first normalizing because either the strings are equal in content and probably equal in form, or they differ early and possibly at some character where normalization is not necessary to determine the result of the comparison, and thus less work need be done to compare strings form-insensitively than first normalizing all inputs (unless so many string comparisons will be done that maybe normalizing first might be a win). For string hashing form-insensitivity is not faster because one has to normalize every input, whereas if all inputs are normalized once then one need never normalize to hash, but still form-insensitivity yields a better user experience.