It is absolutely quick!!

in fact its probably a lot quicker than doing a SUM against the messages

So the reason for the item sizes in EV is for EV Quota Usage and how much data you have taken from exchange, none of the reports will truly reflect on how much data is being used on disk because of single instancing, the size of the DVS, DVSCC, DVSSP etc

But honestly, try a query such as this in a large environment and it will take quite a while
 

SELECT  VSE.VaultStoreName "Vault Store",
        PE.PartitionName "Partition Name",
        PE.PartitionRootPath "Partition Path",
        COUNT(S.IdTransaction) "No. Archived Items",
        SUM(CAST(S.ItemSize AS BIGINT))/1024 "Size Of Items (MB)"
FROM    EnterpriseVaultDirectory.dbo.Archive A,
        EnterpriseVaultDirectory.dbo.Root R,
        EnterpriseVaultDirectory.dbo.VaultStoreEntry VSE,
        EnterpriseVaultdirectory.dbo.PartitionEntry PE,
        EVVSYourStore_1.dbo.ArchivePoint AP,
        EVVSYourStore_1.dbo.Saveset S
WHERE   S.ArchivePointIdentity = AP.ArchivePointIdentity
  AND   AP.ArchivePointId = R.VaultEntryId
  AND   R.RootIdentity = A.RootIdentity
  AND   A.VaultStoreentryId = VSE.VaultStoreEntryId
  AND   S.IdPartition = PE.IdPartition
  AND   VSE.VaultStoreEntryId = PE.VaultStoreEntryId
GROUP BY VSE.VaultStoreName, PE.PartitionName, PE.PartitionRootPath
ORDER BY VSE.VaultStoreName