The latest Firefox build that Debian did only took just over one hour on amd64/armhf and 1.5 hours on ppc64el, the slowest Debian architecture is riscv64 and the last successful build there took only 17.5h, so definitely not days. Your average modern developer-class laptop is going to take a lot less than riscv64 too.
One should never trust the binaries, always build them from source, all the way down to the bootloader.
https://bootstrappable.org/
Checking all the files is really the only way to deal with potential malware, or even security vulns.
https://github.com/crev-dev/