You claimed the unarchiver to be 'rubbish and slow' without offering any reason as to why. On version tracker it's rated 4.4/5 but someone reading this forum might write it off on reading your comment. Hence I kinda probed what you were saying and it turns out you had archived thousands of mp3s.
For your purpose 'betterzip' is indeed better as it allowed you to look inside the archive without extracting it, 'the unarchiver' does not support this functionality and began extracting the archive, which of course is very resource intensive both for cpu and disk hence your system became unresponsive. Having said that, 'the unarchiver' is free and most people don't archive their mp3 collections.
On topic? The op asked for a free utility to which the unarchiver and stuffit expander were suggested, both of which are free. Betterzip is not free. It does in fact seem a good product however if you just want to extract pretty much any archive (and don't make pointless huge archives of thousands of files) then there's no need spending a tenner.
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Oh, and to reiterate to anyone reading this, a simple internet search will reveal stuffit to be pretty much loathed by most mac users.