We tried two last things:
1) We monitored thoroughly network traffic on the server and on the client while running the Spotfire dashboards. We did not see any indication of bottleneck or other issue. Bandwith utilizations varied from 20% to 40%
2) We doubled again the hardware to 16Gb RAM and 8 processors (virtual). There is no improvement in performance
At this stage we feel the performance issue lies squarely in the Spotfire to Matlab off-the-shelf functionality