If you want customers to stay or return to your web site you have to make, sure the performance of your web site is good. If customers have long wait times to use your web site applications they will get frustrated and go to your competitor. You must ensure that web pages download quickly, the web site is available to the customer, and there are few errors or bugs.
Performance can be measured either manually or automatically; it is rather difficult to co-ordinate several customers hitting your web site at the same time. My recommendation is to use an automated testing tool. You need to come up with criteria for selecting a tool, more on a tool selection criteria in another posting.