This is basically a repeat of what other users have asked:
https://www.telerik.com/forums/any-way-to-prevent-auto-loading-when-parameters-are-present
https://www.telerik.com/forums/how-to-disable-report-auto-preview
https://www.telerik.com/forums/how-prevent-report-from-running-on-page-load
The gist of the Telerik team's recommendation is to not provide default values for parameters.
In addition to the rebuttal by the developers (generally speaking, they want to provide default or suggested values to end-users), we have a slightly more generic case. We have users running reports that span months of data. In most cases we have start and end date parameters. The users will sometimes want to see the data from current month AND THEN switch to same month of last year.
They select the start date and the report immediately executes spanning 13 months. The user has to wait until the report is loaded completely to immediately discard the result by setting the end date.
Adding the "AutoRun=false" option would be immensely useful here. Or at least wait for e.g. 5 seconds of user inactivity before executing the report.