What you're asking for is not possible natively within SSRS.
"On-Demand Reporting" simply refers to SSRS being smart about pagination, and not processing any datasets that aren't used on pages that have not yet been rendered. It is already "on", and is a somewhat fundamental bit of functionality in the stack. However, SSRS still needs to completely process any datasets that are being rendered, and if those run slow, the report will too.
Could you use linked reports to jump around between a suite of smaller, faster reports, instead of paging through a single large and slow one?