powershell md5 compare too slow
There are two obvious optimisations.

The first is to remove the line

    $file_dest = Get-ChildItem -PATH $COMP_DEST -name


since $file_dest is unused. I assume that it was previously used instead of

Get-ChildItem -PATH $COMP_DEST -name -Include $file_ori[$i]




The second is to do the remote hashing remotely. Rather than fetch 500 files (of unknown size) across the network in order to calculate their MD5 sums, use remote PowerShell to calculate their MD5 sums and just return the filenames and the hashes. You might even be able to manage to get the two computers to calculate the hashes in parallel, giving a two-fold speedup.