When the question you want to answer requires looking at all or most events for a given source type, very quickly the number of events can become huge. This is what is generally referred to as a "dense search".
For example, if you want to know how many page views happened on your website, the query to answer this question must inspect every event. Since each query uses a processor, we are essentially timing how fast our disk can retrieve the raw data and how fast a single processor can decompress that data. Doing a little math:
1,000,000 hits per day /
10,000 events processed per second =
If we use multiple indexers, or possibly buy much faster disks, we can cut this time, but only linearly. For instance, ...