let's not take the argument into wrong direction
I thinks it's just a healthy discussion on what our experiences have been on the subject and probable causes, I'm certainly not an expert of these things, so I don't take it as an argument, feel free to correct me, I'm here to learn as well.
and every time the results were that there is no issue with speed
From my understanding, speedtest runs a ping, download & upload from your device to the selected server, which is auto selected based on lowest ping. It is mostly your ISP's or nearby ISP's speedtest server and hence the 1 or 2ms ping because of short distance. It is not the most accurate representation of "internet" speed, it just confirms the bandwidth between you and the ISP . It is, however, useful in cases where you want to check any connectivity/bandwidth issues between you & your ISP.
However, there are lot of other ways to speedtest your bandwidth, but it will to & from those specific servers and will present it's own specific results.
For example, google has it's own speedtest servers, ProjectStream
https://projectstream.google.com/speedtest and it will tell you download/upload
speed from google's servers, so it will represent your experience on Google's services like YouTube and Stadia etc.
Likewise, Netflix has it's own speedtest servers at Fast
https://fast.com/# and it will test your connection for their own services.
the issue is UDP packets are getting interrupted
To check connectivity issues to a particular website or server, you have to ping that site from which you are having issues with, run an MTR or Traceroute or Pathping to find out the route from your system via ISP to that server, the number of hops it takes, ping times and packet losses etc. AND THIS IS WHERE ISP's NETWORK MANAGEMENT TEAM COMES IN. How efficient is there routing to different international servers, how diverse is there bandwidth uptake and distribution etc.
It's all up to network management. For example, few years ago we were having connectivity issues at office and through traceroute we found out that our connection to an SEA server in JAPAN was routed and get this, Pakistan-EU-North America-SEA, so we talked to the ISP and got it routed directly.
But regardless, since last few months, I have not been able to find any issues with speed test of bandwidth while there still being these random slow downs to different site with no discrepancies in routing, hence my theory of ISP throttling or bandwidth bottlenecks, I could be wrong.