How can garbage collectors be faster than explicit memory deallocation?

Question

I was reading this html generated, (may expire, Here is the original ps file.)

GC Myth 3: Garbage collectors are always slower than explicit memory deallocation.
GC

Answer 1

A factor not yet mentioned is that when using manual memory allocation, even if object references are guaranteed not to form cycles, determining when the last entity to hold a reference has abandoned it can be expensive, typically requiring the use of reference counters, reference lists, or other means of tracking object usage. Such techniques aren't too bad on single-processor systems, where the cost of an atomic increment may be essentially the same as an ordinary one, but they scale very badly on multi-processor systems, where atomic-increment operations are comparatively expensive.