Vortex Sound Research

Synth1 is probably my favorite free synth, and the main reason it's my favorite is because of the low cpu usage. I've been having issues with it in Ableton lately, whenever I tweak it it doesn't let me pause or play the audio, until I click somewhere outside the plugin. Not a huuuge inconvenience, but it messes with my flow and is leading me to look for a new free low cpu synth. I would like something that can achieve similar results to Synth1, but am also willing to try something a little different. Also, I use a Mac, so I cannot use any Windows only plugins. Any suggestions will be much appreciated thank you.