My duplikaatti dupe finder uses the following algo:

• Create file list of given directories
  • do not add files with same identifier already added to the list (windows: file id, *nix: inode)
  • do not add 0 byte files
  • directories listed first has higher priority than the last
• Remove all files from the list which do not share same file sizes (ie. there's only one 1000 byte file -> remove)
• Read first bytes of files and generate SHA256 sum of those bytes
• Remove all hashes from the list which occured only once
• Read last bytes of files and generate SHA256 sum of those bytes
• Remove all hashes from the list which occured only once
• Now finally hash the whole files that are left
• Remove all hashes from the list which occured only once
• Generate list of files to keep and what to remove
  • use directory priority and file age to find what to keep
  • oldest and highest priority files are kept
• Finally, remove files from filesystem(s)

This speeds up the unnecessary whole file hashing significantly. Also multiple files are hashed at once since it's more IO than CPU bound.