I will assume you mean other devices get over 900mbps download your upload speed is good.
This tends to be some kind of software thing.
Although very unlikely check that nic setting are all defaults. There are setting related to offload features that sometimes cause issue but in most cases the default setting is the best.
Although I don't think you are using brands that load lots of bloatware (ie asus) I would check that you do not have any kind of software installed that talks about favoring one kind of traffic over another. These many times pretend they can reduce network latency in games. But any kind of QoS software can cause issue and should not be used/installed.
Have you tried different browsers.
Although it will just prove it is some windows setting but not tell you which you could try to boot one of the many linux USB images. These run from the USB stick and have a browser installed so you can run normal speed tests. This though will just verify the hardware any maybe any strange issue with your router or internet.
Since you have a second machine you can also try a very old line mode program called IPERF. This is very simplisc program that is not affected by things like browsers or most other software. It will verify that your nic settings and drivers are correct. You should get over 900mbps in both directions. Again though this will just tell you that there is some setting someplace in windows causing the issue not what it is.
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