Minimum Game Speed

Aside from a very small number of gimicky rush decks that have been found so far that can potentially kill in the 150-180s range, I have noticed that there is another magic number for game speed that has shown up.

-A mana stockpile/ direct damage to opponent deck has been shown that runs in about 240-250s
-I’ve been able to work out over a dozen or more creature rush decks that use 1-3 resource types to spam 1or2 creatures that all have an ideal kill time of 240-300s
-I’ve seen a couple variants of Direct Damage/Creature Rush hybrid decks that can kill in an ideal time of 280-300s
-240s allows for 5 card draws (unless the deck is specialized to get extra draws fast) Meaning most decks will be 12 cards into their strategy (which is a decent amount of time to set up defenses for a conventional deck).

What I’m wondering is if the fact that the range 240-300 shows up so much with the current cards suggests that perhaps we should consider a minimum game time of something between 240-300 to be a good guideline.
What would that guideline be useful for? Well for balancing, if a proposed deck is able to reliably win a game in less than the guideline, it suggests that the mechanics&cards involved in the deck may need to be examined.
Conversely, if a new “killer” deck or combo is created that takes a lot longer than the guideline, then it is probably safe to presume the combo is not unbalanced at first glance.