Question Speedtest Download Speed slowly diminishing on desktop


Oct 23, 2016
Hello everybody !
I am not a networking expert, but I wanted to ask about something I found really interesting about my internet speed on desktop I built.
I am using this wifi adapter on my desktop (TP Link TL 881 ND), I downloaded latest driver (dated 2018-02-07 , lol).
When I'm doing a speedtest on my iphone 11, it is around 11-13 mbps, I remember it was also similar with ethernet cable.

But when I'm doing speedtest with my new desktop, it also starts with 11-13 mbps at the beginning and falls down to 3-4 mbps at the end of the test, ending with download score of 4mbps.
Does anybody know what might be causing this ? Like is there some firewall setting thats causing this ?

Thank you very much !

edit: Another update that may help. When I open the resource monitor in task manager--performance tab while on speedtest I see under network tab that it is also 13000 kb and speedtest says the same. However it suddenly drops to 0-50 kb/sec sometimes causing it to look like im getting 4mbps while in reality its probably the average. Can someone tell why im going down to nonexistent download speeds randomly ?

edit 2: After disconnecting 2-3 devices and restarting the router, i was able to get a solid 13mbps on desktop (like other devices). Theres generally 6-7 devices connected to wifi. Although I have no idea whats the number of devices my modem can support, I think it shouldn't cause such a problem.
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