Ah, got it, sorry if I was explaining what you already knew. Definitely not for gauging client speed as you already knew, but also not for even testing true uplink speed from a given piece of infrastructure. Just for others reading/browsing, I'd go so far as to say that the Dashboard throughput test isn't really a "throughput" testing tool the way we'd tend to think of it.
Results are going to vary based on many things including the product, the model and firmware version, naturally the load on the device, which shards they're connected to in which data centers, heck maybe even the time of day and the weather. Just kidding on the last part.
I will make it a point to discuss this with the Dashboard team. I would like the "Throughput" test tool to be renamed to better reflect what it's actually doing... Which I would say is testing the control plane communications to Dashboard.
I didn't write the tool and never saw the code, but I'd imagine there are several "it depends" types of factors. And results can be confusing. For example, I just ran a test on my lab MX65 which got 50Mbps, then an MS120 behind the MX65 which got about 20Mbps, and then an MR42 AP hanging off that switch which got 110Mbps. I have also seen cases where a mesh repeater AP reports higher throughput test results than its corresponding gateway AP.
Anyway, I'm on it, I'll discuss this with the Dashboard PM team at one of the monthly meetings and report back. If it goes several weeks and I haven't checked back, ping me with a reminder.