SpreadsheetWEB uses a caching algorithm to cache workbook calculations in the memory of the server, and it stays in the memory if that workbook is in use. If the application is not in use (after a 10 minutes timeout in Software-as-a-Service SaaS environments), then SpreadsheetWEB removes it from the memory. That means next time a request comes to that workbook, it needs to be loaded in the memory first. During the loading page and calculation transitions will be slower. But if the workbook is fully loaded in the memory, page transitions and calculations will take only a few seconds for all users. Note that once a workbook is cached it serves all users. Hence loading is not per user but per server.
Note that caching timeout applies to SaaS environments where thousands of users access these servers. If you have a dedicated server environment caching timeout can be disabled. As a result, your application will always load faster.