Regression on openflow REST performance test

We have detected performance regression on the openflow REST config test:

https://jenkins.opendaylight.org/releng/view/openflowplugin/job/openflowplugin-csit-1node-config-performance-only-boron/plot/Flow%20Config%20Performance/

The regression is quite severe, from 200 flows/sec (single flow/POST transaction) to 4 flows/sec and it seems it is been introduced after this patch (master is also impacted):

https://git.opendaylight.org/gerrit/#/c/47980/

To reproduce:

• Start controller with openflow feature: odl-openflowplugin-flow-services-ui
• Start mininet linear: sudo mn --controller=remote,ip=192.168.0.1 --topo linear,15
• Start perf test: python odl_tester.py --threads 5 --flows 20000 --timeout 10

odl_tester.py script is available in int/test repo: test/tools/odl-mdsal-clustering-tests/clustering-performance-test