I reverted to the ‘legacy’ poller-service.py service (from librenms-service.py) and it performs much better.
I have actually realized I raised something similar ages ago:
It looks like the problem is still there to some degree - and performance of librenms-service.py got somehow worse in recent releases.
Seems like this thing is getting confused by Nexus routers as well a bunch of UCS 6300 FIs - disabling them brings polling back to acceptable levels.
This project could really use high-performance poller with in-flight window tuning on per-device basis, like the AKIPS one (https://www.akips.com/showdoc/blog2) - I would have contributed the code if I had any idea how to implement that 
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