Seven tile subdomains configured, still only six connections

• At most 6 requests sit in `Pending` while 40 or more rows are greyed out as `Queueing` during a drag
• Actual request URLs are `https://tile.example.com/{z}/{x}/{y}.png` with no `t0` to `t7` prefix
• First-screen tiles take 3 to 4 seconds, barely better than 5 to 6 seconds without subdomains

1. Check whether real request URLs contain a `t0` or `t3` prefix
2. Group requests by domain in the network panel and count active hosts
3. Run `nslookup t0.example.com` and `nslookup t1.example.com` to see whether they hit the same IP
4. Check the response protocol and `server` header for HTTP/2
5. Verify the URL template literally contains `{s}`

Enable HTTP/2 (`listen 443 ssl http2`) so the six-per-domain limit stops mattering, or point subdomains at different backends and use `https://t{s}.example.com/{z}/{x}/{y}.png`; cutting request count with `tileSize: 512, zoomOffset: -1` also helps.

L.tileLayer('https://tile.example.com/{z}/{x}/{y}.png', {
  subdomains: '01234567'      // 配置了但未生效
});
L.tileLayer('https://t{s}.example.com/{z}/{x}/{y}.png', {
  subdomains: '01234567'
});

# Nginx：启用 HTTP/2 并给瓦片加长缓存
server {
  listen 443 ssl http2;
  server_name t0.example.com t1.example.com t2.example.com;

  location / {
    root /var/www/tiles;
    expires 30d;
    add_header Cache-Control "public, immutable";
  }
}

# DNS：不同子域指向不同节点，避免单机排队
t0.example.com  A  10.0.1.11
t1.example.com  A  10.0.1.12
t2.example.com  A  10.0.1.13

• Adding `subdomains` without `{s}` in the template, so the option is silently ignored
• CNAME-ing every subdomain to one IP and expecting a speedup, which just moves the queue to the server