For those wondering where this feature has gone, please see the below, which I posted in another thread:

This feature used to be available via the app for Deco XE75s however has been removed in later versions of the app.  I know as I was able to perform these tests when I first purchased the XE75s.

It is now only available to certain Deco models per this article:

https://www.tp-link.com/uk/support/faq/1462/

However, apparently the reason why it has been removed from some models is due to this statement in the article:

NOTE:

1. The speed test result shows the speed that the main deco gets from the modem, not the speed on the client connected to the deco.

Apparently users complained it was misleading so the developers removed it from certain models but not all, which in turn makes no sense whatsoever.

I am in the camp that this is importanly functionality and this feature should be reinstated across all Deco models and have fed this back directly to tp-Link via their chat support.