I feel like the definition of stall is a fairly straightforward one for most top tier competitive players. However, there's probably a strong case to be made now that people nowadays use "stall" and "slow" interchangeably, which is moronic.
Stalling is a tactic by which the player/team's main goal is to "not lose". This is very different from the concept of "winning". Stall can be used to classify certain playstyles or teams, but to generalize an entire generation into a playstyle is simply idiotic.
Stall DOESN'T MEAN slow in any sense of the word. GSC and ADV are slower gens, but they're not stall unless you consciously make them play that way. Stall by definition means inability to make progress. This is ultimately your own doing, stop blaming the gen.