BreeceW
Load the list of people who reacted to a message before mouse hover
This is not a possibility (loading without user input) because that would require making a separate request for every reaction. What I can try, however, is removing the “Who reacted…” placeholder, though that would remove any indication that it is loading. Let me know if you can think of a more pleasant way to indicate loading before showing who reacted (or even a completely different UI—it doesn’t have to be a tooltip).
Yes, I think that's definitely a good improvement if you cannot get the reactions list in advance
The way this works today is it loads the people who reacted once the tooltip opens. To mask the delay, you could make it load on pointer over (with a slight delay in case of accidental hovering), that way, the tooltip content would be ready by the time the tooltip opens.