multithread GC more slower?

[zsxxsz]

I start two threads, one is producter and the other is consumer. I found if GC 
was enable, the time cost was 24 seconds, and if GC was disable, the time cost 
was 4 seconds. Why is GC so slowly in multi-threads?
Please see the attatchment.

module test_cond;
import core.sync.mutex;
import core.sync.condition; 
import core.thread;     
import core.sys.posix.pthread;
import std.c.stdio;     
import std.c.stdlib;            
import std.c.string;    
import std.c.time;              
import std.stream;      
import core.memory;     

private struct Job
{       
	Job *next;
	int   i;
}       

const bool gc_disable = true;

void test_cond()        
{                               
	Mutex m_mutex = new Mutex;
	Condition m_cond = new Condition(m_mutex);
	Job* m_jobHead = null, m_jobTail = null;
	File fp = new File("out.txt", FileMode.Out | FileMode.In);
	int  max = 1000000;
	time_t begin, end;

	static if (gc_disable) {
		GC.disable();
	}               
	void threadProducter()  
	{                               
		for (int i = 0; i < max; i++) {
			Job *job = new Job;
			job.i = i;
			m_mutex.lock();
			if (m_jobHead == null)
				m_jobHead = job;
			else
				m_jobTail.next = job;
			m_jobTail = job;
			m_mutex.unlock();
			m_cond.notify();
		}
	}

	void threadConsummer()
	{
		char buf[256];
		m_mutex.lock();
		for (;;) {
			while (m_jobHead == null) {
				m_cond.wait();
			}
			Job *job = m_jobHead;
			if (job != null)
				m_jobHead = job.next;
			m_mutex.unlock();
			snprintf(cast(char*) buf, buf.sizeof,
					cast(const char*) "doJob thread id: %u, n: %d\n",
					pthread_self(), job.i);
			fp.writeBlock(buf.ptr, strlen(buf.ptr));
			m_mutex.lock();
			if (job.i == max - 1) {
				static if (gc_disable) {
					delete job;
				}
				break;
			}
			static if (gc_disable) {
				delete job;
			}
		}
	}

	Thread consummer = new Thread(&threadConsummer);
	Thread producter = new Thread(&threadProducter);

	begin = time(null);

	producter.start();
	consummer.start();
	producter.join();
	consummer.join();

	end = time(null);

	static if (gc_disable) {
		printf("not use gc: write over, time: %ld\r\n", end - begin);
		GC.enable();
	} else {
		printf("use gc: write over, time: %ld\r\n", end - begin);
	}

	delete fp;
}