It's likely that you have some system that is not finishing within the period of 5 minutes. Also you have only a single thread of the poller running. Try this:
*/10 * * * * root /opt/observium/poller-wrapper.py 4 >> /dev/null 2>&1
Replacing the "1" with "4", or you might try 2 and see where that gets you. Further to see which devices are taking a long time check the polling stats. Click on the globe in the menu then "Polling Information". Scroll down the list to see which of your devices is taking the longest.
Lastly, you may want to add a setting to you config.php to set the max-reps for snmpbulkwalk:
$config['snmp']['max-rep'] = 5; // allow use of -Cr in snmpbulkwalk vastly increasing walk speed
Again, start low and move up. You might want to try manually polling the slow device from the cli and playing with that number to find your optimal setting.
I just went through the same issue and the combination of these things resolved my problems.
Hope that helps!
--chip