Web apps consist of two parts: front-end and backend. Front-end lives in a browser instance, so you don't need to verify front-end itself for more than one users, while backend you should -- it will collect data from many users.
It may also happen this monitoring "thread" is so resource-consuming that it interferes with main functionality of the app, e.g. you are not able to fill in or scroll document.
In sum, I would perform three types of performance tests:
- Load test backend with monitoring and non-monitoring requests and check its response time: JMeter
- Load test backend with monitoring and non-monitoring requests and check how responsive frontend in the browser is (subjective application responsiveness): JMeter + you playing with the app
I guess also that buffering strategy (how much data can be buffered before sending them) might be an input variable in those tests, e.g., your developers might be curious what buffering strategy is the best, so I would play with multiple different strategies.