First answer of the vmware team was in June 2014 when they acknowledge this issue.
Now we are in september 2015 and the performance issues are still exactly the same and the C# Client was not improved as well.
I agree with everything which has already be said, especially that you should dig a hole and bury this web client. I am even not sure if a new htlm5 web client from scratch would be a good solution.
Please, improve the C# client. The web client will always be slower no matter what you do.