it's easy to check specific speed benchmarks with almost any stat calculator so i'm not sure if you're looking for an answer that's so specific. for the sake of completeness, you need precisely 114 base speed, with 0 investment and a neutral nature, to outrun a base 70, 252+ pokemon. but the significance of EVs is an interesting topic.
without a doubt speed is the most powerful out of all the stats when it comes to EV investment. like it or not, speed is the only stat where one point can make all the difference. that's why speed creep is so powerful. it's often almost completely inconsequential to sacrifice one point of any other stat, but if you drop that point in speed and come up against a "standard" set running an unenhanced speed investment, suddenly you outspeed and an entire new world of possibilities opens up. without a doubt, speed is the stat that appreciates investment the most.
as for the other stats though, EV investment is usually mostly for assuring chances. it becomes magnified if the attack in question is super effective or if you use a boosting nature. investing enough EVs in attack stats can change 50% chance to ohko to a 100% chance and indeed sometimes the game swings on those tiny little things. however the significance of small investments in defenses or attack is generally minimal in a vacuum. the impact adds up over the course of a game, undoubtedly, but this is why throwing a small random amount of EVs in attack is pointless for a defensive pokemon, or vice versa. it doesn't really accomplish much in the specific scenarios where you expect said pokemon to perform.
it's also important to remember that the EV system rewards concentrated investment. especially when multiplicative factors like nature or typing are taken into account. spreading your EVs across several stats, even if all those stats are important, can often detract from a mon's general usefulness. i used to run a blissey spread of 252 def / 44 satk / 92 spd / 120+ sdef. yeah. the 44 satk and 92 speed assured me to outspeed any 8 speed scizor and guaranteed an ohko on 248/8 scizor with fire blast. in exchange for this extremely specific scenario, blissey loses quite a bit of special bulk. when you have to switch it into rain backed politoed hydro pumps multiple times, that disadvantage is really gonna add up. another example from one of my old teams was mix support tyranitar with 252 hp / 76 atk / 60 satk / 120+ defense (relaxed). this assures a 2hko on uninvested latios with undoubled pursuit and assures an ohko on 252/0 gliscor with ice beam, but tyranitar's bulk loses out massively, especially its special bulk which i chose not to invest in for that set (but it could take a scarf Landorus-I's hammer arm!). all those specific investments cut down on the overall long-term utility of the mon, which in both cases was being able to switch in and tank repeatedly. in exchange you get very specific advantages upon which you have to gamble quite a bit of EV investment so you have to decide if those things are really as important as they seem.
anyway your question was a bit open ended so i hope this helps?