Starting a new java process is indeed slow.
Unfortunately, porting PlantUML in C# is a huge task and there is no plan for this in the future.
However, there is a solution to improve general speed.
The idea is to run PlantUML as a daemon, so that you do not have to launch a new Java process for each diagram modification.
This may sound weird but you can run PlantUML as FTP server:
This has been successfully used in CodeUML:
I think it can solve your performance issue.
Keep us informed on your progress on this side!