Learn German: Mein Fuß rutscht blitzschnell auf der Stufe, obwohl ich aufpasse. - My foot slips lightning-fast on the step, although I am paying attention.

blitzschnell is an intensifying compound made of the noun der Blitz (lightning) and the adjective schnell (fast). While German has many established combinations like this, such as steinhart for rock-hard, you generally should not invent your own. Stick to the common ones native speakers use. Notice that in this sentence, it is functioning as an adverb describing how the foot slips.

Why is it auf der Stufe (dative) and not auf die Stufe (accusative), since slipping involves motion?

auf is a two-way preposition. We use the accusative for a change of destination (moving onto the step) and the dative for a fixed location (an action happening on the step). Even though your foot is moving, the entire action of slipping is taking place on the surface of that one step. Therefore, it takes the dative feminine der Stufe.

Why isn't the separable prefix separated in obwohl ich aufpasse?

obwohl is a subordinating conjunction, which means it forces the conjugated verb to the very end of the clause. When a separable verb like aufpassen is pushed to the end of a subordinate clause like this, its prefix (auf-) and the main verb (passe) reunite into a single attached word.

Could I use the separable verb ausrutschen instead of just rutschen here?