That’s an awesome speed. That’s fast enough to stream at least 15 videos simultaneously. So that’s not the problem.
I was just mucking around with a YouTube guitar teaching vid and had no problems. This morning, some stupid cat video on FB was churning like mad. Beats me.
You haven’t changed the channel yet? Wifi interference can be quite intermittent, depending on what your neighbors are doing. Change the channel and see how it does for a day or so. If it’s still not good, change it again. You have three channels to choose from.
To be clear there are more than three channels to choose from but (usually) you really only want to use channel 1, 6 or 11.
That could change though if a neighbor chose a channel other than one of those three. This is where using a wifi analyzer can help.
Apparently my wifi channel is 149 
. And I see no way to change that in ‘properties’.
Channel 149 is for 5G wifi (channels 36-165 are for 5G). The 5G spectrum is usually far less crowded but 5G is impeded much more by walls and such than 2.4G is.
Channels 1-11 are for 2.4G wifi. For 2.4G you want to pick either channel 1, 6 or 11.
Interesting. I’m beginning to think that this all may be due to operator error, as in getting ahead of the streaming when trying to find something in a video. My guitar instructor was doing a lot of jumping around in a video last night and we had no stalls; probably because it had enough time to completely download.
FYI YouTube videos (generally) do not download while you wait. They make sure a bit more than you need is there then it stops. When you start playing it tries to stay just ahead of you. If you stop it stops sending data.
Pretty sure Netflix works this way too.