There are a couple of possibilities:
First, are you using the M1 (arm64) build of R? You need to do that before RStudio will start using native mode:
https://cran.r-project.org/bin/macosx/
Secondly, only the parts of RStudio that work with R itself are native (arm64) code. Activity Monitor will show you that the main RStudio executable (rstudio) is Intel, but the process that actually connects to R (rsession) should run native code once you've installed R for arm64.
To be clear, the new release doesn't provide native versions of all RStudio components; it just lets you run the native version of R inside RStudio so your R code runs at full/native speed. Hope that helps! We're working on a fully native version of RStudio and hope to have it available later this year.