I'm assuming the slowdown is caused by Eset scanning each and every file that is "opened" by Backup Exec as it is backing them up, which is why excluding the Backup Exec processes/folders isn't making a difference. Weird though as i would have thought Backup Exec was just copying the files, which i wouldn't have thought counted as opening them.
Have you tried excluding the backup destination in case it is scanning files as they get created there?
Also is it perhaps possible to use Eshell (as someone suggested in the other thread) to somehow disable "Scan on File Open" and then scheduling it to toggle off for the backup?