I've tried many newsreaders, Forte, News Leecher, Newsman Pro, newsbin, Agent, Newsrover, Grabit, hogwasher, etc. The best luck I've had so far is with Forte, saving the posts I want as nzbs and loading them unto sabnzbd, newsman pro worked great since it saved and showed large posts in a collapsable thread, and it automatically saved them unto a specific folder without specifying, so you could easily download an entire newsgroup. It's too bad that it was extremely slow, and very unefficient. One big problem is that downloading/par2/unrar from usenet is hd depedent, so that might be why it slows your computer down to a crawl, you can try putting par2/unrar unto a storage drive so this doesn't happen(or getting an ssd). I know some of these are not available on mac, but maybe this will be of some use to you.