We never noticed any significant performance difference between Java and .NET PD4ML implementations. Most probably your HTML documents have some structures, whose rendering has been optimized in the recent Java versions: as you may see the actual .NET version is quite older than the most recent Java version.

Currently we’ve been working hard on the version synchronization and we plan to release a .NET port of v380 in about one week. Hopefully it will resolve the issue you faced with.