Is it possible to enlarge memory usage for Simulink

I have a massive model (lots of 'save to workspace', 'scope'), which only simulate at normal speed in the 1st run. Simulation speed reduces to 20 step/sec after 2nd run.

I have to do 'clear all' before rerun to regain the speed.

I understood OS (I am using uBuntu 64-bit) has memory limit on each process. And I have enlarged Java heap size to 4GB. (I have 16GB in total). Doing such sent away memory error, but doesn't help with Simulation rerun.

I've monitored system memory usage, its doesn't seem to go beyond 4G while simulation is running. ubuntu `ulimit save 'unlimited', 'max memory size ulimited'. Is there a limit in Simulink?

Because 1st run is good at speed, I don't think it's processing power issue. At least 2 cores are running at 100%. Not much disk activity. I am already using the latest i7 CPU, and don't assume I need fast SSD because of disk activity.