Hello,
%ports.ifHighSpeed is the link speed, not the used bandwidth.
Maybe you want to take a look to the actual Port utilisation over threshold rule included in LibreNMS.
If you dont have it, just click in Create rule from collection.
It is based in percentage from link speed. So, if you have a 1G port and set the rule to trigger at 80%, it will do when bw usage reachs 800Mbps.
Then, if you want to alert when that 80% of usage happens for 30 consecutive minutes, you need to set delay to 1800s.
Interval is the time between alert notifications to transports.