Its simple...
You compare them to their respective averages.
For example, when we were comparing Fire Emblem Lords, Tibarn was banned cause he's too good, despite falling under the technical definition (Leader of an Army w/ a Unique Job.) The person I was discussing this said Lyn may stand a chance a since she'd avoid a doubling and has bows to nail Tibarn's 3x weakness, I said that wasn't necessarily the case, we compared and Tibarn does indeed Double Lyn. naturally, we weren't going "Tibarn is 40 FE Speed, Lyn is 29 FE speed!" or whatever; that's just silly! We did it more like this:
Tibarn is 11 Points above average speed relative to his game. Lyn is 7 points above average speed relative to her game. 11 - 7 = 4. You need 4 points to double in an FE game, therefor, Tibarn doubles Lyn.
Eirika, meanwhile, barely avoids a double, being 8 points above Average, thus only 3 below Tibarn. However, she still loses cause while Siegliende hits a weakness, she can't avoid counters the way Lyn could, and that weapon lowers her speed below the threshold.
So yeah, raw speed doesn't matter, its still relative speed. You just simply do Character's Speed vs. their respective average, then compare the two. So a +4 AS speed doubles a perfectly average FE character, even if their numeric speed is identical (this matters mostly for FE10 vs. FE6-9)