SNMP request can take too long if the number of SNMP request retries and timeout are set to high for the policy used to access the device.
Go to 
> Settings > Control Panel > SNMP Policies and check the parameters Retry and Timeout for the policy used on the device. You can see witch policy is configured on the device by going to > Settings > Control Panel > Devices.
For more information, go to chapter Configuring SNMP Policies.