Restful API,the TPS Slow down

Question

My simple object like this:

pom.xml

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>1.5.10.RELEASE</version>
</parent>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <exclusions>  
            <exclusion>  
                <groupId>org.springframework.boot</groupId>  
                <artifactId>spring-boot-starter-tomcat</artifactId>  
            </exclusion>  
        </exclusions>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-undertow</artifactId>
    </dependency>  
</dependencies>
<!-- Package as an executable jar -->
<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

The Class like this:

@RestController
@SpringBootApplication
public class Example {

    @RequestMapping("/")
    String home() {
        return "Hello World!__";
    }

    public static void main(String[] args) throws Exception {
        SpringApplication.run(Example.class, args);
    }
}

Execute the command:

/home/gdct/jdk1.8.0_152/bin/java -server -Xms1g -Xmx1g -XX:MetaspaceSize=128M -XX:MaxMetaspaceSize=256m -jar /home/gdct/abc/aembedjar.jar --server.port=9998

At first,the TPS is 3600,But After a few minutes,THE TPS is 1500.(Slow down)

Please help me,how to keep the TPS is 3500(or other number) or so ???

note: I had test a webservice api, it can keep the tps.