that's how it works: project navigation and VSTs may slow down the DAW. cure is to increase the buffer (and latency as well) if you need to. it's not specific to Reaper or Mac, I've seen this a lot in other programs.
two years ago I asked Reaper devs whether that's normal and Justin said that he definitely has UI rendering and audio playback in different threads, so it's more likely matter of computer not able to handle it all or some non-reaper related bugs. here's
the link