Unless you’re using a really complicated and/or slow hash, loading the data from the disk is going to take much longer than computing the hash (unless you use RAM disks or top-end SSDs).
So to compare two files, use this algorithm:
- Compare sizes
- Compare dates (be careful here: this can give you the wrong answer; you must test whether this is the case for you or not)
- Compare the hashes
This allows for a fast fail (if the sizes are different, you know that the files are different).
To make things even faster, you can compute the hash once and save it along with the file. Also save the file date and size into this extra file, so you know quickly when you have to recompute the hash or delete the hash file when the main file changes.