This might be a more general question than just neon.

Yesterday I checked the speed of my internet connection using www.speedtest.net. I ran the speed test a few times. Each time I got speeds of just shy of 1 gb/s. However, at the same time the speeds indicated by System Monitor at best barely broke 1 mb/s.

Can I not trust System Monitor?

I think this is a valid question. Because; I got different results from plasma-systemmonitor in my tests at www.speedtest.net.