7 tiles is 2 matches (1 bomb, 1 regular match), 8 tiles would be worse as it requires the additional blasting of the bomb, which will often trigger def heroes to fire first (in addition to not being able to use the bomb to make that second match)
Avg is 3 matches (unless you can make 2 bombs in 2 matches and then blast it, which is not impossible, but also not very likely).
Of course playing fast heroes requires different board play than playing avg heroes does. You can´t just jump in and make the first 3-match you see and hope 2 more will pop up. If you don´t have a bomb ready to make, you need to play away the off colors first while setting up the dragon bomb. Then you make the bomb and hope for 1 more match 
Of course doesn´t always work, but I would say 3 out of 4 cases fast is 2 matches. If it´s fast enough is of course a whole different story, but if fast isn´t fast enough, then avg sure isn´t.
It also depends what kind of troops you have, if you got all the 5 star troops you need, then avg is fine, because your heroes are so tanky, you don´t care if the defense fires first, you take the punch and then murder them when you are ready, but if you have to play with 4 star troops, well you´re usually dead by that time.
I don´t know how to explain that really, sorry. I always thought this was obvious, maybe I should try to come up with some good way of explaining this:
I write this as a not-top-100 player for not-top-100 players because SE is basically irrelevant for top-100-players. It´s hard enough for me to find something relevant, anyone playing higher than me can pick a bench warmer for the museum, they´re not likely to ever max or use a hero from this (or any) SE selection, besides Ludwig in rush war lol.
Well I don´t consider there to be much difference between the bottom end of top100 and where I am so don´t start nitpicking, if you´re in 87th ranked alliance and you have like 5 heroes from SE maxed.
let´s say it in a way that can´t be misconstrued. No hero in SE will ever be a meta hero. There you go. So people that think about meta, have no reason to look the SE way. better off using 20 5ers for feeders for their next project than to pick one from here.
Those people still prefer fast heroes, but they can easily get away with using avg speed heroes, becaus all their stuff is LB2 with a big fat 5er troop below.
For the likes of me, 9 tiles is the absolute bottom end of what is playable, it´s not great, but it´s sometimes (more like often) necessary for lack of alternatives. But only with a headache before even hitting the fight button. 3 matches in my world can be an eternity. Every tile faster than that is a shimmer of hope on the horizon. usually still ends in death these days… but what are we gonna do about it
OK so I don´t know if that helped at all or was just useless rambling on my part, but it´s the best I can come up with on short notice.
