可以使用Java的线程池来控制多个实例的并发执行，从而减少等待时间。

下面是使用Apache HttpClient 4.5的示例代码：

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpRequestBase;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;

public class HttpParallelExecutor {
    
    private static final int MAX_TOTAL = 200;
    private static final int MAX_PER_ROUTE = 20;
    
    private HttpClient httpClient;
    private ExecutorService executorService;
    
    public HttpParallelExecutor() {
        PoolingHttpClientConnectionManager connManager = new PoolingHttpClientConnectionManager();
        connManager.setMaxTotal(MAX_TOTAL);
        connManager.setDefaultMaxPerRoute(MAX_PER_ROUTE);
        this.httpClient = HttpClientBuilder.create().setConnectionManager(connManager).build();
        this.executorService = Executors.newFixedThreadPool(MAX_PER_ROUTE);
    }
    
    public void execute(HttpRequestBase requestBase) {
        executorService.execute(new HttpTask(httpClient, requestBase));
    }
    
    private static class HttpTask implements Runnable {
        private final HttpClient httpClient;
        private final HttpRequestBase requestBase;
        
        public HttpTask(HttpClient httpClient, HttpRequestBase requestBase) {
            this.httpClient = httpClient;
            this.requestBase = requestBase;
        }

        @Override
        public void run() {
            try {
                HttpResponse response = httpClient.execute(requestBase);
                // 处理响应
            } catch (Exception e) {
                // 处理异常
            } finally {
                requestBase.releaseConnection();
            }
        }
    }
}

使用该类来执行GET和POST请求：

import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;

import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;

public class Main {
    
    public static void main(String[] args) throws URISyntaxException, IOException {
        HttpParallelExecutor executor = new HttpParallelExecutor();
        
        HttpGet httpGet1 = new HttpGet(new URI("http://example.com/get1"));
        HttpGet httpGet2 = new HttpGet(new URI("http://example.com/get2"));
        HttpPost httpPost1 = new HttpPost(new URI("http://example.com/post1"));
        HttpPost httpPost2 = new HttpPost(new URI("http://example.com/post2"));
        httpPost1.setEntity(new StringEntity("Hello, World!"));
        httpPost2.setEntity(new StringEntity("Foo bar baz"));
        
        executor.execute(httpGet1);
        executor.execute(httpGet2);
        executor.execute(httpPost1);
        executor.execute(httpPost2);
    }
}