If you right-click on a track in Reaper, go to the Render/Freeze Tracks menu item, and from there you can choose to render or freeze tracks and mute the originals. This applies all of the effects on that track and renders it to a new audio file, and mutes the old track. Now you can hear the effects, but they aren't being processed real-time, and therefore don't cause any CPU usage (well, no CPU usage above what a normal audio track would incur).
You can use Ctrl+Click to select multiple tracks before right-clicking if you want to render/freeze multiple tracks at the same time.
Or you can select one or more tracks and go to the File->Render menu. At the top of that menu, select "selected tracks" instead of "master mix", and you can render stems from there. That'll accomplish the same thing, only with a little more control of the file format and file names of the resulting files.