Unlabeled icons are terrible. I use Gmail more than any other application outside of dev tools like my editor and my terminal, and I use every single one of the message operations (archive, snooze, add as task, report spam, etc.) at least occasionally. After a month with a new work Gmail account I realized I was still waiting for the tooltip to pop up before clicking the buttons, just to make sure of what I was clicking on. Fortunately, Gmail lets you switch to text buttons. Now when I want to snooze a message, I just click snooze, instead of thinking, "This button is a clock, I bet it's snooze, but hold up, what if it's another time-related operation I'm not thinking of... okay, confirmed with the tooltip, now I can click."
And that's for a button I click several times per week!
I still use the html gmail, that's getting harder to access all the time (and which google now just straight up ignores being set as your default view, and gives you the javascript view anyway unless you manipulate the URL)
The convention for mobile UIs is that tooltips show up upon long pressing the buttons. Unfortunately, sometimes that does nothing, or something else entirely (e.g. start dragging).
And that's for a button I click several times per week!