If code runs slowly, the output of code chunks can be cached, so next time the document is rendered the existing cached results can be used.
The caching system in R markdown can be activated by setting the chunk option
The caching system can be tricky to set up for complex projects, and the
targets package is a much more robust framework for building reproducible analysis pipelines.
See this demonstration project for an example of how to set up
If you need a template for many reports, for example, showing results for a specific location or time period, consider using Paramertised reports which allow you to pass a variable into an R markdown document when it is knitted.