"But... But I want to be a big karate cyborg... ;_;" - Nik Hasta
"Get off my lawn! ...on this forum, that just makes people think of Cyclops." - Sharpandpointies
"...makes me think the Night King just says "Screw the rules, I have magic money" when it comes to physics." -Captain Morgan
The feat is not to be taken seriously but not for the reason you are stating. Do you really think the writers thought about the speed of clouds moving when that scene happened? Your just over analyzing a scene. The writers also didn't think about the speed of lightening and just thought it would look cool, that's all. Similar to Cap holding back Thanos' arm; a cool moment. The reason to not take it seriously is because it's way past any other speed feat she has and would be considered an outlier.
In Cap's case, it looked more like Thanos stopped out of curiosity than any actual strength feat on Cap's part, especially since Thanos had just casually smacked aside Cap's physical peer Black Panther in his vibranium jammies beforehand. It was like he went, "Did this small adult or possibly large toddler really just try and stop me with brute strength? That is almost as adorable as it is sad."
I have also done so, and my feeling is that Thanos was surprised Cap temporarily stopped his hand...because he wasn't trying hard but still didn't expect Cap to manage against the casual exertion of force. Kind of more like 'I am surprised this toddler stopped me from closing my fingers. Fascinating. I will now actually try a little. Ah, there we go.'
Why are we here?
"Superboy Prime (the yelling guy if he needs clarification)..." - Postmania
"...dropping an orca whale made of fire on your enemies is a pretty strong opening move." - Nik
"Why throw punches when you can be making everyone around you sterile mutant corpses?" - Pendaran, regarding Dr. Fate