Evolution and stat growth

Stats are based on four things:

-Base stats of the Pokemon species, which are fixed values for each species
-Individual values (IVs) of the Pokemon, which are values from 0-31 for each of the six stats and cannot be modified
-Effort values (EVs) of the Pokemon, which are values from 0-31 for each of the six stats and can be modified
-level

The formula for calculating stats basically looks like (base stats + IVs + EVs) * (level/100). It's actually a little more complicated, with some multipliers, but that's the gist of it. When you evolve a Pokemon, the only thing that you change is the species, and thus the base stats. So it doesn't matter when you evolve it - the end stats will be the same. For your example question, Scizor would still lose speed after evolving at level 100.

Effort Values are maximum of 255 per stat, usual capped at 252, with 510 total EVs gained overall.