CustomHeightmapTerrainProvider pegs CPU

I’m trying to troubleshoot bad performance when using CustomHeightmapTerrainProvider.

I do not see the same issue when using Cesium.createWorldTerrain().
but as soon as i simply add the following, i see CPU usage go high.

viewer.terrainProvider = new Cesium.CustomHeightmapTerrainProvider({
            width: 256,
            height: 256,
            callback: function (x, y, level) {
              return new Int16Array(256*256);
            }
          });

Thoughts?

Here is a Sandcastle to show the problem.

Example