Basic connection speed is a good thing, but latency (ping time) is more so once you achieve a certain basic speed (which yours is well above for most needs). You ping times to the speed test site look fine, but when you have issues with services, try pinging them to see how they look, test end-to-end. The speed test is only really testing a fairly limited set of paths & systems, and I suspect ISPs give those sites higher priority.
For a bad car analogy, your car can do 200mph on a clear motorway but when you try to get to Lands End via the twisties full of caravans, it goes slower. The internet at large can be the the twisties, the speed test site is the motorway destination.
Then you have a whole other set of variables you have no control over like how much capacity does the service you are using have. It's finite of course, and things like Zoom are going to be hugely busy right now. I've no idea how they manage their system nor how it works, but when the client reports "unstable internet" it can mean pretty much anything, but probably that it is having trouble connecting with the host site, but that probably has nothing at all to do with your setup or anything you can do anything about.
A common tool used by bad actors to mess with internet services is a DOS - denial of service attack. Basically they swamp a service with requests to the point it becomes unusable. As a client of that service, the symptom from your end would look like a problem with the internet connection...until you check somewhere else and find all is well with your connection.