I am not denying that good receivers usually have good three cone times. I am merely pointing out that there are and have been good slot players with poor three cones.
And my greater point is that it's kind of weird to me to judge Allison on his combine workout this far into his career. We have actually tape of him playing in the NFL.
In thinking about productive receivers who lacked athleticism Anquan Boldin came to mind: 4.73 / 7.35 at the Combine.
Landy and Boldin have certain commonalities: tough, physical, durable, YAC by throwing off tacklers. Allison does not fit that profile. And he also doesn't fit the waterbug type. Edelman is a name I see thrown around in this dicussion. Welker is the prototype running short inside routes getting YAC on separation. Welker and Edelman accumulated a lot of concussions along the way which might explain why Belchick has typically had a couple of these types on the roster.
We do have Allison tape, like this highlight tape from 2018:
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I don't believe there's a slot route in this tape; one play is hard to tell. On the subject of long speed it's hard to tell which play he hit that blazing 20 mph mark. Maybe the one at 1:23 after he regains his stride around the 15 yard mark. It's probably not one where he's looking back for the ball.
The question is whether any of this matters. Do you actually need a Welker/Edelman ankle breaker or a Landy/Boldin physical guy for underneath routes? Allison lacks quickness and physicality. What he brings is sharp route running, good hands, adjustment to the ball, big catch radius, long speed.
So, even if Allison lacks the characteristics of a prototypical slot receiver of the various types, the question is, "Does it matter?" in the Lafluer passing offense that features stack and bunch formations with combination routes. It's a concept that works off DB hesitation, rubs and picks with crossing routes, then switch it up with the play action downfield routes. The idea is to dull the DBs twitch. It's a group concept rather than a one-on-one concept where the Allison skill set can be a slot fit; the concept is that the scheme as much as the player creates separation.
Further, if you're running a cross out of a bunch or stack, is the wideout vs. slot distinction even meaningful? No, it isn't.
So, what about those short under routes in the danger zone which Tom Brady will tell you is fertile ground with the rule changes making it a little less dangerous? LaFleur might have other things in mind: TE, RB release, or rotation for matchups.
We await the results.