Terrain tiles load slowly and the first screen spins for over ten seconds

• globe.tilesLoaded stays false for more than 12 seconds.
• RequestScheduler.statistics shows numberOfActiveRequests 6 and numberOfPendingRequests 231 out of 412 attempted.
• terrain requests sit pending in the network panel while imagery is already complete.

1. Read Cesium.RequestScheduler.statistics to separate concurrency limits from slow server responses.
2. Measure a single terrain tile, which should take 10 to 30 ms and weigh a few tens of KB.
3. Compare total request counts with requestWaterMask on and off.
4. Check the response Cache-Control, since without max-age every visit refetches.
5. Check whether HTTP/1.1 caps the origin at 6 connections.

Create the provider with requestWaterMask: false and requestVertexNormals: false, set globe.maximumScreenSpaceError to 4, or start at 8 and lower it to 3 after 2.5 seconds, and raise tileCacheSize from 100 to 200. Serve tiles over HTTP/2 with Cache-Control public, max-age 31536000 and immutable.

console.log(Cesium.RequestScheduler.statistics);
// { numberOfAttemptedRequests: 412,
//   numberOfActiveRequests: 6,
//   numberOfCancelledRequests: 0,
//   numberOfPendingRequests: 231 }

var terrain = await Cesium.CesiumTerrainProvider.fromUrl(
  'https://terrain.example.cn/tiles/',
  {
    requestWaterMask: false,
    requestVertexNormals: false
  }
);
viewer.terrainProvider = terrain;

• Raising the concurrency limit as the only fix: bandwidth is split thinner and the last tile still arrives late.
• Leaving requestWaterMask on for an inland project, doubling every tile request for water that never appears.