We are working on adding a flow monitor that can analyze the network traffic on a network switch or router that has support for either netflow, s-flow or j-flow. We don't have an ETA for that yet, but it will be released in a future update for KNM v5.
Other ways of measuring "slowness" is dependant on what the network infrastructure is capable of. Some switches for example can send SNMP traps when you reach some threshold. It's difficult to say exactly how to do it because of the number of vendors and models out there so I would have refer you to your switch's or router's manual.
Regards,