With Splunk, you can search through enormous volumes of data (proportional to the ever growing number of events to summarize as well as the number of users accessing the data), potentially increasing the amount of time it takes to complete beyond acceptable lengths.
Generally speaking, you create summaries of the data to report on big data. These summaries are created by background runs of the search upon which your report is based. When a user runs a report against data that has been (pre-) summarized, it runs considerably faster because the summaries it uses are much smaller bites of the total number of events to be searched.
This concept (of pre-summarizing events) can be used with your reports, data models, and indexes. Right now, we will focus on report summary acceleration.