by Jim » Wed Sep 05, 2012 9:35 am
That database is very small, not even 2 megabytes in size, and should cause no slow down in a store. (I have seen stores with databases over 200 meg that still function quickly.) The SKU field is indexed so looking up an item by sku should not be a problem.
Any slowdown in adding an item to the cart would not be caused by looking something up in the database. The add to cart form/link has all that is needed to add the item to the shopping cart. The database would not even be accessed unless you have inventory tracking enabled, in which case a check is made to see if there are enough items in stock for the order.
Things that might slow the cart loading are accessing a realtime shipping carrier like UPS, USPS, FedEx or a custom shipping API. Also if you have a custom tax API configured or use Avatax that might also take some time. Also if you have Google Analytics enabled it will have to contact the Google server to record that a transaction is inprocess. If you have other features (ads, etc) that load content from other sites that would also cause a delay.
Usually the biggest problem would be that the store is on a server that is overloaded. Some of the low cost hosts put thousands of accounts all on a single server. That much load could cause a problem but it would normally not take more than a couple of seconds to add an item to the cart.