Is this a new problem?
Is it the same devices which are not getting polled in time, or all devices?
Did you try increase the threads like poller-wrapper.py 16 ? ( Performance - LibreNMS Docs)
Since the poller is working, but just not improving i would be looking elsewhere
Try run atop on the box and check to see if the cpu is waiting on disk.
Run an MTR trace to some high polled devices to see if you have some issues in the network.
Do you have any resource limits specified in your docker-compose files & check for load stats of different containers
Try run an snmpwalk from the librenms vm, then try it from a different box ideally outside that environment, measure it with the time command. See if it’s quicker 
Also check this don’t know if its related; The poller has not completed within the last 5 minutes, check the cron job - #13 by Brian_Gibson