There would not be any performance set back as such. This is done by checking SHA 1 checksum value. For sure there would be there would be some extra condition checking not that would not impact performance as such.
//If the asset you upload already exists in AEM Assets, the Duplicates Detected dialog warns you that you are attempting to upload a duplicate asset. The dialog appears only if the SHA 1 checksum value of the binary of the existing asset matches that of the asset you upload. In this case, the names of assets is immaterial. In other words, the dialog can even appear for assets that have different names if the SHA 1 values for their binaries are the same.
I hope this would help you.
Thanks and Regards
Kautuk Sahni
Kautuk Sahni