The number of marks being shown in a sheet will directly impact the performance of that sheet.
Most of the dashboards are meant to provide a summary of detailed data to our audience. What is the point in having millions of data points in a dashboard
(I guess, EXCEPT, when trying to find outliers or create a density map )
so, in every situation where you can reduce the # of marks to only what you need, you will see significant change in the amount of time it takes to create that view
The previous tip I wrote about this highlights is very well. The next tip is on the Tableau extracts