A sales journal will reduce the time consumed in journalizing and posting entries to the ledger?

Yes, that is true. Otherwise, every time you made a sale, you would have to show the sales and the reduction in inventory. Consider if you owned a store. If someone came in and bought eggs, bread, milk and juice, you could have eight ledger entries to make (sale x 4 items, then inventory/cogs for four items, then reserve for return x 4 items, then cash or accounts receivable for 4 items. With a sales ledger, the items are all grouped on one receipt and the total for that transaction is reported.