Qlikview Server 9 performance issues with AJAX ZFC

[#1]

We are in the middle of performance testing QVS9.

Our test uses 20 qvw files that were duplicated from the same file. The file size is 1.1MB and the UI has one table and one listbox.

With this small data and UI we were expecting to see pretty good performance for 20 simultaneous users but the response times are pretty slow.
It takes 15 seconds on average for the page to load (including the AJAX data calls) and about the same time to filter or sort data (so any click on the page takes 15 seconds where the page is "stuck")

We are using the new qlikview web server for the tests on an eight core box with 4 GB memory (although the CPU doesn't get passed 10% usage and there's plenty of free memory, so I don't think that's the problem)
We tried using IIS but it just errors out when we run the tests.

Is anyone else experiencing performance issues with the AJAX UI?

[#2]

We have similar problems as well - running zfc, we see slow response times using the workbench on a 800MB file. Sometimes the responses come back in milliseconds and others come back taking seconds - it is very random throughout the day.

Did you come across any leads to the cause of your issues?