Cryptographic hashes are not the fastest possible hash, but they are not slow; CPUs have hardware SHA acceleration: https://www.intel.com/content/www/us/en/developer/articles/t... - compared to the likes of a password hash where you want to do a lot of rounds and make checking slow, as a defense against bruteforcing.
That sounds even harder; Windows Authenticode uses SHA1 or SHA256 on partial file bytes, the AV will use its own hash likely on the full file bytes, and you need a malware which matches both - so the AV will think it's legit and Windows will think it's legit.
That sounds even harder; Windows Authenticode uses SHA1 or SHA256 on partial file bytes, the AV will use its own hash likely on the full file bytes, and you need a malware which matches both - so the AV will think it's legit and Windows will think it's legit.