My iMac 2015 5K, 32GB RAM, SSD APFS instrument drive / High Sierra is freezing as well. Happens after reloading song, rebooting Cubase 9.5.10 and rebooting iMac. After about 20 seconds, I have access to Cubase, but this happens very consistently. In fact, this issue arose with only a couple of MIDI tracks and VSTi’s. No such issues using 9.x. Looks like time for another update.