Note: After using the command, the original wallet.dat will be renamed into the backup file and the "new" wallet.dat is the salvaged wallet.

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Rescan shouldn't take "forever" (days?) to complete with the average hardware unless the wallet has too many keys/transactions in it, should only be hours at most.
My guess is your Bitcoin Core have the "prune block storage" settings enabled which will require it to re-download and verify everything from start every time you load a new wallet.dat.
If you mean, "a few hours", it's normal.

Check the settings if it's enabled (Disable: Settings->Options...->Prune block storage to).
After disabling it, Bitcoin Core will use more disk space (about 400GB).