Why is CPU on the universal forwarder not used most of the time and we see error "Could not send data to output queue (parsing queue), retrying...."?

Hi, I was monitoring Universal Forwarder's CPU usage with the environment below, and I put 13GB sized file on Universal Forwarder server to send to the indexer and monitored it with limits.conf set maxKBps = 1024. I noticed that most of the time, CPU isn't used, however, every 30 seconds it jumps up to about 10%. I checked splunkd.log on the UF to see what's going on, and saw this error Could not send data to output queue (parsing queue), retrying.... and at the same time, the the summary count on Indexer goes up. It seems like the Universal Forwarder is sending data to Indexer only every 30 seconds? Is there any interval for the UF to send data, or is the Indexer controlling UF from sending the data? Indexer: OS - Windows Server 2012 r2 Splunk - 6.3.2 Universal Forwarder: OS - Centos64 Splunk - 6.3.2