Throughput degrades over time

From comment in INDY-223:

I run load test with -c 20 -r 50000 parameters, 1.000.000 requests total. My script checks largest objects every 5 minutes, sysstat checks memory every 10 seconds.

I checked it after ~18 hours - only ~220.000 transactions were processed, whereas first 10.000 were processed in a first 10 minutes.

Same thing I saw when run test_node_load test from plenum/test/test_performance.py.