Latency. Just tested with a "hello world" page (18ms ping).
Chrome takes 102-200ms to render the page, Firefox takes 150-400ms. 1.5 to 2x slower -- and that's after all the performance upgrades they've done with Rust and Quantum.
Network inspector says Firefox is spending 150-270ms "blocked" before it even initiates the connection. On Chrome the block is 3-18ms (15-50x (!!) faster).
There's also UI lag. When I press Ctrl+N, I expect a new window to open immediately. Chrome and Chromium based browsers do that. I cannot even perceive the delay between pressing Ctrl+N and a new window appearing. With Firefox it takes about 0.5-1 seconds before anything happens at all. This seems to be another 10-20x difference (though I'd have to record my screen and measure the difference).
Curiously, Re: UI lag, I think the actual time until the navbar is usable is about the same. But Firefox waits until the whole window is built before it displays it, while Chrome opens the window immediately and fills it in. From a user experience (I do command, I expect feedback) this makes a huge difference.
I switch back to Firefox roughly once a year after reading how fast it's gotten. But I have been disappointed on this front every single time. I want Firefox to succeed, I really do. I am donating to Mozilla every month. But as it is right now, I can't tolerate such lag, for a trivial action I perform thousands of times a day.