If you’re looking for fast queries of OpenStreetMap data, check out GeoDesk. You can turn any OSM PBF file into a GOL (a geographic object library – a compact single-file database stored on your own system). There are SDKs for Python, Java and C++, as well as a command-line utility – all free & open-source. A query to retrieve every w[power=line] in Germany should execute in less than 100 milliseconds on a modern machine.
GeoDesk doesn’t support KML as a native output format, but there are options for emitting OSM data as WKT or GeoJSON. Using the toolkits, it should be fairly trivial to write a converter that turns the Java/C++/Python objects returned by a GeoDesk query into the XML format of KML.