The compressed version is the Supporter release, so only saves from the uncompressed supporter release should be fully compatible.
The release on the OP however is the Steam version, which is not the full All-Languages release that the Supporter release is, so it may or may not be compatible because of that
[compatibility status pending community feedback]
The biggest reason for the difference in performance is that in the compressed version, there is no encryption.
Any performance difference between the 2 versions on my post
here is the compression. Both are decrypted.
UAGC has a tendency to completely lock up my system while it's processing, so I haven't been wanting to see if the latest UAGC version improves anything further.