When a request is sent by a user for a report to Reporting Services, with default configurations, it needs to extract data from the related source, process and then render. Depending on the volume of data, the complexity of the process and the time it takes for forming the output, it might take few seconds or it might take hours for rendering. If it takes a longer time than expected, it is considered as a performance issue.
Caching improves the performance. Reporting Services can cache the report with initial execution based on the instructions given by us and use the cached copy for stratifying next requests on the same report with the same parameter values if it is a parameterized report. Caching does not help with the first request. For an example, if it takes 5 minutes for opening the report, the next request for the same will be treated from the cache, hence request will be satisfied within a few seconds.