如何优化 Go 语言编写的 Web 服务以提高并发处理能力？

package main

import (
    "fmt"
    "net/http"
    "time"
)

func handler(w http.ResponseWriter, r *http.Request) {
    time.Sleep(2 * time.Second) // 模拟处理时间
    fmt.Fprintf(w, "Hello, World!")
}

func main() {
    http.HandleFunc("/", handler)
    http.ListenAndServe(":8080", nil)
}

func handler(w http.ResponseWriter, r *http.Request) {
    go func() {
        time.Sleep(2 * time.Second)
        fmt.Fprintf(w, "Hello, World!")
    }()
}

func handler(w http.ResponseWriter, r *http.Request) {
    ch := make(chan string)
    go func() {
        time.Sleep(2 * time.Second)
        ch <- "Hello, World!"
    }()
    fmt.Fprintf(w, <-ch)
}

import (
    "database/sql"
    _ "github.com/lib/pq"
)

var db *sql.DB

func initDB() {
    var err error
    db, err = sql.Open("postgres", "user=username dbname=mydb sslmode=disable")
    if err != nil {
        panic(err)
    }
    db.SetMaxOpenConns(10)
    db.SetMaxIdleConns(5)
}

import "sync"

var cache sync.Map

func getFromCache(key string) (string, bool) {
    if value, ok := cache.Load(key); ok {
        return value.(string), true
    }
    return "", false
}

func setToCache(key, value string) {
    cache.Store(key, value)
}

import (
    "net/http"
    "net/http/httputil"
    "net/url"
)

func main() {
    backend1 := "http://localhost:8081"
    backend2 := "http://localhost:8082"

    url1, _ := url.Parse(backend1)
    url2, _ := url.Parse(backend2)

    proxy1 := httputil.NewSingleHostReverseProxy(url1)
    proxy2 := httputil.NewSingleHostReverseProxy(url2)

    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        if r.URL.Path == "/service1" {
            proxy1.ServeHTTP(w, r)
        } else if r.URL.Path == "/service2" {
            proxy2.ServeHTTP(w, r)
        } else {
            http.Error(w, "Not found", http.StatusNotFound)
        }
    })

    http.ListenAndServe(":8080", nil)
}