When you run the flowgraph, can you look at system monitor? This will give some indication whether the problem is that all the cores are pegged or if RAM is filling up.
A couple of other things to look at:
1) Is there any text being printed to the console?
2) What happens if you disable the GUI blocks? Simple method would be to open the flowgraph select the blocks that have GUIs associated with them, hit D (for disable) and then run the graph.
3) Would you happen to know which SIMD instructions it is using? Run
gcc -dM -E - < /dev/null | grep -e "SE" -e "AVX"
The gcc -dM -E will print out the preprocessor defines. The - < /dev/null forces gcc to exit immediately. The "SE" flag filters the preprocessor defines for any of the SSEE SIMD instructions; the "AVX" flag filters for the AVX SIMD instructions.
Sometimes the processor supports SIMD instructions that the compiler (due to age) does not support.