I just did some testing with du on my Windows machine. This does "work", but it is not fast - I doubt any faster than File::Find. It visits every file and adds the sizes up - this takes awhile! Its a lot slower than the "properties" click button in the Windows file manager. Satisfying both "fast" and "multi-platform" simultaneously is a pretty tall order.
maybe if we look at your File::Find code, we can see some improvements?
Update: This is the fastest way that I know how, that also satisfies "multi-platform". This code runs on my Windows machine.
find (\&sum_bytes, "C:/temp");
return() unless (-f $File::Find::name);
$byte_total += -s _; # not a typo!
# "_" different than "$_"!
# see [id://951317]
print "total bytes in C:/temp: $byte_total in $file_total files\n";
# prints on my Windows system:
# total bytes in C:/temp: 656201485 in 2554 files
One "expensive" file operation is run for each file system entry. The results of that "file system query", "stat" operation are re-used in a subsequent file operation. This is multi-platform, but not completely optimal for Windows NTFS.
You can get "fastest" and you can get "multi-platform", but not both together. There is a Windows NTFS way to get this number faster. But this is the "fastest" multi-platform solution of which I am aware.
File::Find has to visit all files as well... And while I don't claim to have any Windows related knowledge, I do know that on Unix, you'll have to visit all files (or rather, their inodes) to calculate the "size of the directory".