Which are you using (e.g., V2RayN, Shadowrocket, or command line)?
The simplest solution is to switch to a faster and more reliable DNS server. Public DNS services like Google's DNS (8.8.8.8, 8.8.4.4) or Cloudflare's DNS (1.1.1.1, 1.0.0.1) are popular choices for their speed and reliability. v2ray slow dns server