You are misunderstanding this paper deeply. Quoted summary from the paper itself:
>>>We use this framework to compare
the time-space performance of a range of garbage collectors to explicit memory management with the Lea memory allocator. Comparing runtime, space consumption, and virtual memory footprints
over a range of benchmarks, we show that the runtime performance
of the best-performing garbage collector is competitive with explicit memory management when given enough memory. In particular, when garbage collection has five times as much memory
as required, its runtime performance matches or slightly exceeds
that of explicit memory management. However, garbage collection’s performance degrades substantially when it must use smaller
heaps. With three times as much memory, it runs 17% slower on
average, and with twice as much memory, it runs 70% slower. Garbage collection also is more susceptible to paging when physical
memory is scarce. In such conditions, all of the garbage collectors
we examine here suffer order-of-magnitude performance penalties
relative to explicit memory management.<<<
GC is "competitive" when is has 5x as much memory according to the authors. Well, probably this is because GCs are fast when they have low to no memory pressure. They simply don't need to garbage collect anything. In fact, a non-collecting, non-managed, memory approaches can be faster than any memory management. That is, you have a short lived application that just allocates what it needs, never frees anything, then dies when done. But this is not apples to apples.
The paper also says that with 2x more memory than manual management, GC is 70% slower and under paging scenarios GC can suffer "order-or-magnitude performance penalties relative to explicit memory management."
Except under unusual conditions, there is no case where GC is faster than manual.