Your KML file has over 3 million coordinates which is hitting some upper limit for a single KML file in Google Earth. You can scale to that number of contours/features and more if you break it up into smaller KML files, load them in a parent KML via NetworkLinks, and use Regions to only load those features that are in view.
Google Earth can scale up to millions of points and features but not all at once.
If the 15,439 features are geographically separable then you can break up the KML by some geographic grouping or fixed gridded area each in its own KML. Maybe 15 KML files each with 1000 features might be a good starting place.
The parent KML should have each NetworkLink with the appropriate Region and Level of detail (Lod) element to prevent all of the KML files from loading all at once.
Here's the structure of the parent KML file:
<kml xmlns="http://www.opengis.net/kml/2.2">
<Document>
<NetworkLink>
<name>area 1</name>
<Region>
<LatLonAltBox>
<north>xx</north>
<south>xx</south>
<east>xx</east>
<west>xx</west>
</LatLonAltBox>
<Lod>
<minLodPixels>32</minLodPixels>
</Lod>
</Region>
<Link>
<href>1.kml</href>
</Link>
</NetworkLink>
<NetworkLink>
<name>area 2</name>
...
</NetworkLink>
...
</Document>
</kml>