When running a speed test you’ll need to factor in the upload limitations as a possible point of interference. If another device on your network has Microsoft Onedrive, iCloud, Dropbox, ect. Running in the background and using up all the upload speed, this can cause issues with other devices trying to use the download aspect of your connection.
For example, let’s say you want to watch a YouTube video. Your device first has to send out or “upload” a request to start playing that video. When the video starts playing the video file hasn’t fully loaded yet. The device has to keep sending out or “uploading” requests for the next segment of video to load. This is why services like YouTube TV, Netflix, Hulu, ect. are referred to as “Streaming Services”, they are sending your device a constant stream of video segments. In order for these streams to work correctly they need constant responses from the device to let it know when to send out the next segment. So if one device on your network is taking up all the upload, all the other devices are going to reflect a poor download speed.
Below is a link for a very useful software you can use that limits network connection speeds for both upload and download.