So, I'm going to preface this with noting that familiarity is the single largest factor for typing speed- at least when it comes to key board types. And a lot of it could also be a matter of confidence, as well.

In any case, the prevailing understanding is that mechanical keys are faster than membrane keys because of how they work. membrane keyboards are basically what you have on your microwave- a circuit board or some substrate with part of a curcuit, and a polymer membrane that has the other side of that circuit. You complete the circuit by squashing down on a blister in that membrane, and it registers a key press. Chiclet keys on most laptops are the exact same, except for the key pad you see riding on top of the blister.

Mechanical keyboards, on the other hand, are spring loaded, and the key press is registered somewhere between the top and bottom of the key's motion. exactly where depends on the switch, mind. The OG mechanical keyboards from way back in the day, were all clicky- the point of the click mechanism is that it would tell you when the key press was registered. You could hear and feel it as you pressed down and you could let without fully pressing the key down.

This greatly reduced the fatigue a typist would feel, letting them type more. it also improved typing speed. (this was, in point of fact compared to typewriters, at the time. but it's still true compared to chiclet keyboards.)

fast forward to the modern era with gaming keyboards going back to highly customizable switches. Linear switches (like Cherry MX Reds, have a progressive resistance to the key press. The further you push it down... the stronger the spring pushes back. There's no tactile mechanism- clicky-clacky or just 'tactile' clicky without the klacky.

while people can feel the point where the press is registered, that is not me, so I wind up bottoming out each key stroke- which again slows me down and causes more fatigue to build up.

As a side note, this is also good for gaming because in some games, you can ride the key on either side of the point it registers and squeak out a lot of rapid presses, which is harder to do with a click spring up in there.

Things like the cherry MX browns, that are tactile without being super loud about it, are better, but I still type faster on the full clicky-keys. (kahlil whites, IIRC.)

Part of me wonders about hall effect keyboards, but I can't be bothered to spend that much money just yet.