The latest Preview (Version 1.2.1206) and the previous both use an enormous amount of CPU. It looks like RStudio uses around 30% CPU and the underlying QtWebEngineProcess uses around 50% CPU for EVERY instance of RStudio that is running.
When RStudio is started, this happens immediately and the laptop's fan starts working hard.
I couldn't find any reports on this, which is strange as this is a showstopper for using the program :-/
I'm using a MacBook Pro, 16GB RAM, macOS 10.14.1 Mojave.