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!