Generally the higher your upload speed, the smoother the measurements. Some devices and browsers are better or worse than others at displaying a smooth upload speed.Buffer Bloat can also make for a bumpy upload speed.If your PC uses one or more SSD (flash) disks, it could be 2 to 8 gigabit.
Since the test does use some graphics, those may also be a source of excessive CPU usage in cases where the browser is not well suited to the OS..
Check whether the drivers for your computer are the latest. If your ISP is offering a near 100megabit download speed and your desktop ethernet card is 100mbit, you may find the ethernet driver is not capable enough to handle the load.
If an extension in the browser can interrupt a download or upload, edits the page in some way, or requires that it be checked before running, then it can cause the speed test to return errors or not run correctly.
You should be fully aware of what you have installed on your PC and know what it is doing!
In speed test preferences you can set a maximum memory use, for example 10, for 10 megabytes.
The test will then use much less memory but it may not drive a connection to full speed.
You can post in the feedback topic or if you're chasing gigabit speeds try the gigabit feedback topic speedtest gigabit testing/results/debugging or if you are interested in Buffer Bloat, there is a buffer bloat topic too:
Or you can send me an instant message via the site.
If you are interested in ping time to all different locations, the speed test has a latency radar plot. Although it tries not to exceed the limitations of your mobile device, it might.