GCP Cloud SQL database export takes a lot of time

You can see Cloud SQL for SQL server as a Google-managed virtual machine that within it has a SQL server instance.
As you have already noticed, part of the export is to first back up the database and then upload it to a Cloud Storage bucket.
If the backup is taking 3 hours out of the total 12, then that tells me that the remainder is time mostly spent on uploading the backup file from the Cloud SQL vm instance to the Cloud Storage bucket.

As you well know high-speed internet connections are asymmetric — they are designed to provide much better speed for downloading than uploading.
Importing from a Cloud Storage bucket does not involve backing up the database, so this means 3 hours less in comparison with the export.
What I am trying to say is that you cannot expect the same operation duration for Cloud SQL exports and imports.

Uploading 8 terabytes in about 9 hours is the same as uploading 14 Gib per minute; this transfer rate can vary as it is managed by Google because Cloud SQL is a Google-managed product.
If you require a higher transfer rate or an option to configure it, you can open a feature request here.

As of now to improve the Cloud SQL export operation time you can follow the best practices.
You can also make sure that the export bucket is in the same or closest region to your Cloud SQL instance; that should reduce the number of hops the backup file needs to go through.