I haven't completed enough tests to provide specific data points, but I have found that when I stack two groups of 4* Knights, one to generalize the Glass Cannons (Cepha, Bianca, Violetta, Casablanca, Dandelion), another for the Tanks (Di Yu, Snowflake, Cyani, Hibiscus, Sweetpea) and pit them against the Bosses of 12-5 (and getting them there at full HP is no cakewalk), as well as Raid Bosses, although no group could reliably take them down, all of them dished out roughly the same amount of damage.

Needless to say, the Cannons dished out more damage in a shorter span of time, while the tanks dolled it out over a longer period of time (from 50% to nearly double the time). One advantage the tanks did have though was more opportunities to activate their skills, but currently, the difference is minimal. Rather than the numbers, like a few others above me, I also heavily emphasize stacking abilities that compliment each other (and also had similar results when stacking Atk/Def abilities); it definitely nets me more damage in the Raids rather than throwing random teams together.

And then, as those that have her would know, Water Lily is also completely defiant of her power level. Numerically, she is currently the weakest 5* Knight and by all rights, should be the first unit to die every-time. Except, she's usually one of the last ones standing and even dishing out a boatload of damage at the same time, thanks to both her Defense boost and Evasion ability, on top of having a 32~37% Skill Activation + a high base Attack to make use of it. So yeah, that just further reinforces my habit of making teams based around abilities instead of Power Level.

As for speed, I usually have one fast group and average every other group, but definitely prefer having one super fast and super slow group to rely on; they tend to make clearing the event maps a million times easier.