Like every other language used for game development: you don't allocate objects in the render loop. All objects are re-used and go into pools. Malloc isn't free (heh): maintaining the free lists can kill FPS just as quickly as anything else.

As others have pointed out: the CLR has had a concurrent low-latency GC for a long time now and now that it's all open-source there is nothing preventing Unity from adopting it.