The file containing the recorded Capture rate is called /var/log/csr-history/network_txt, or for past files network_txt-YYYYMMDD.gz.  There are many lines per interface and it requires some knowledge to read.  What is needed to get a quick look is to have Linux parse through and summarize the statistics.  The provided parsing commands will assist in bringing copious amounts of information down to the interesting details.  If you don't have access to the CLI, you can locate the network_txt files in the Customer Service Report (CSR) downloadable from the Settings > System page in the GUI.
 

The system records the capture rate every five minutes in the network_txt file.  To get the Capture rate in megabytes, sorted with highest last run:

grep ^bytes network_txt | grep -vw 0| awk {'print $3}' | sort -n

Sample output from this command will look like the following:

3,852,644
3,852,644
4,150,253
4,150,253
4,447,690
4,447,690
4,652,156
4,652,156
5,009,763

This pulls the lines beginning with bytes (bytes/second), removes those interfaces which have no traffic, and then displays column three in numerical order, largest last.  This number is bytes per second.  The numbers are duplicatd because the file displays an aggregate capture rate and then repeats the rate for the specific interface.  The result can be searched in vi by taking one of the returned numeric values and searching for it.  Then look a few lines above for the Date and Time of when it was recorded.  You can also search down to locate the interface where the value was recorded.

This next command does much the same thing but instead of printing the exact capture rate string, it multiplies times 8 so that the result is in bits instead of bytes.  It does not yield a number searchable in vi.

grep ^bytes network_txt | grep -vw 0| awk '{sum = $3 * 8; print sum}' | sort -n

Sample output will look like the following:

5528
5528
5528
6272
6272
7288
7288

To get the statistics from previous logs, change the first grep to zgrep and the filename being parsed to include the earlier messages files, like messages.1.gz or messages.2.gz.

With the more recent releases, the system records the capture rate every minute.  This allows the system to see the capture rates with better granularity.  The command to run as root is:

grep eGBpd messages | sort -t: -k 11,11n | tail -25 |awk -F: '{print $NF*1024*1024*1024*8/86400/1000/1000/1000}'