Look at the instruments and don't worry about clouds

I would also say it is due to the fact, that the clouds are simply not detailed enough. Basically, a SIM cloud is a huge blob of white whatever, being close to the cloud, you completely lose all details. If you would have a comparison to the real deal, you would need to find a cloud which is basically 4, 5 times bigger without additional details. This, along with the sprite character, completely abolishes the real perception of flying through clouds. Easy "test": fly in the mountains with low level clouds. You will realize that the perception of speed in regard to the clouds seems to be slow, while the perception of speed in relation to the ground is ok. Simply because the ground offers enough details to really perceive the speed, while the undetailed clouds do not.

I would venture that it is partly down to the mixture of "3D" clouds and 2 dimensional sprites that are constantly facing the viewpoint in the sim. Because they swing around to stay facing you, the edges of the sprites move unnaturally and this detracts from the sense of speed. That and the fact that clouds fade out relatively far from the viewpoint as you fly through them.

 

That's pretty much it. Even with 4096x4096 cloud textures, they lack structure and definition. They look fine from a distance, and even close-up, but the sense of movement and speed is lacking.

IMHO it has nothing to do at all with details and resolution (or the lack of peripheral vision): when you are flying through clouds at a speed of 300 kts and look 90 degrees sideways, so straight out of the side window, the clouds should pass by at a terrifying speed. Even if they are utterly low res. But they don't. They pass by as if you are driving a bike. Really very slowly.

I think it has to do with the clouds not being actually 3D but sprites that turn around with your view. We all know things in the distance move by slower than things nearby. Because we are never actually flying through 3D clouds but always past flat sprites there always is a distance between us and the cloud we think we are seeing in the sim. We THINK we are flying through clouds but we are looking at sprites which always are at a distance and that's why clouds pass by slowly, unlike actual 3D objects we see during take off and landing. If clouds in the sim will become real 3D the problem will be solved instantly.

 

EDIT

Just checked and the clouds that I SEEM to be flying through pass by at the same speed trees do during landing at a distance of a few hundred meters. So the cloud seems to be close but in the sim world it is a sprite at a distance of a few hundred meters only giving the impression it is close by because of the size. We are fooled into thinking the cloud is nearby but it isn't. The speed of 3D objects passing by during landing is spot on so the problem cannot be low res textures or too little detail: it is the distance of the actual 'cloud' in the sim world: because it is a lot further away than we think it passes by so slowly.

All of the above.  There is a distortion in the view, things in the middle of a view do not move as fast as they should across the arc of view.   There is less detail to "grab hold of" with  your eyes.  The sprites are acting at a distance from the cockpit, I imagine so as not to clip/bleed into it.  That last one is the big one.  You can't get your eye on anything easily, but when you can, it doesn't come near you,it  doesn't fly over your shoulder, and your perspective of it does not change because sprite. It's like somebody walking around you at half the free throw line; the ball never comes at you.  They can all be fixed, but for that we are probably at the limits of 32bit before we get there.  (architecture student before becoming a computer geek, stupid CAD Lab)