At the very heart of SharePoint are lists. Just about everything in SharePoint is a list. It can be stated that SharePoint content is list driven.
One of the shortcomings of the previous version of SharePoint was that performance began to degrade if a list had more than 2000 items. This doesn't mean if a list had 2001 items, the performance degradation was noticeable. A SharePoint list could have millions of records. Let's break down exactly what happens when a user requests information from a list.
A user clicks on a link to see the items in a list. It invokes a query back to SQL to pull those items and present them. The query looks at how many columns are on that list and does a "select all" operation. The more columns in a list, the more data that is returned, and the longer it takes to select all of that data because it is number of rows multiplied by number of columns.
Resource throttling is a set of configuration items built into the software...