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At home I have a wireless network with a broadband router being the access point. Using my laptop, I get speedtest.net results of
~25ms ping time, ~40 Mbps download speed, ~13 Mbps upload speed
My PC, however, has had all sorts of connectivity and speed issues; I just replaced my wireless adapter, and am connected, by the speed test results are
~25ms ping time, 2.5-4 Mbps download speed, ~12.5 Mbps upload speed
what could cause this to happen? I mean, if it were all slow, I'd understand, but how can the upload be fast while the download is slow?
Here's a shot of the result of running mtr for a couple of minutes, in case that helps: 
Note: Not a dupe of this question, in which the speeds did not differ for different devices.
einpoklum
Posted 2016-10-01T15:36:27.757
Reputation: 5 032
Obvious consideration your question does not address - is there any other activity on the network at the time of testing? If something else (such as your laptop, TV, etc.) is hogging the download BW the computer will not get much. – Ecnerwal – 2016-10-01T17:45:12.153
@Ecnerwal: No other activity, and, at any rate, the tests were made within 30 seconds of each other with nobody doing anything on other computers. I also repeated the tests a couple of times (which is why for some values I gave a range). – einpoklum – 2016-10-01T18:14:20.273
What do your tests look like plugged into the router as a reference? Alot of things could cause this. Additionally, what is the make & model # of your router? And what is the make/ model # of the wireless adapter in question? – Tim_Stewart – 2018-02-09T18:06:16.197
@Tim_Stewart: This question is from over a year ago, I don't see this anymore. – einpoklum – 2018-02-09T18:40:38.383
Close the question? Your still hitting the unanswered list – Tim_Stewart – 2018-02-09T18:56:10.967