There seems to be some confusion around how this works. First off the ability to delete messages is off by default.
The IT Admin who setup the Teams environment will need to turn on the admin ability to delete messages. Once they have enabled the functionality, admins will be able to delete user messages.
If your IT Admin as enabled it and you still are having troubles please let me know in the comments below.
This item is actively being worked upon.
There are several dozen separate update pieces being worked upon that will provide an overall performance boost. As these updates get closer to completion I’ll provide a timeline as they prepare to roll out.
There is a lot of good progress, as I get more details I will share them. The survey was a huge help in pin pointing how we can best serve you with these updates.
This update is specifically targeting the Giphy selector scenario and the memory improvement is seen in that case. We have numerous memory improvements in the works which all work to improve the overall memory usage. Unfortunately I don't have any details I can share at this time. However, I will post more updates as soon as I am able.