Back in Gen IV, being at or above base 100 speed was great. But now, there are more incredibly fast Pokemon, like the new Megas, Talonflame, and Greninja. 100 base speed just doesn't cut it this gen. What is a good speed for a Pokemon to be at to be considered "average" or "above average?"
100 base speed could definitely still be considered good. It could out-speed many average Pokémon with ease if its stats are invested correctly, or even if not.
Mega Pinsir, Pidgeot, Garchomp, the three Simi's & Mega Glalie are only a handful of Pokémon that have 100-105 base speed and they could out-speed just about anything in their designated tier.
However if you're looking for a good speed stat among the fastest of Pokémon then about 115-120 + would be you're basic good.
With Mega Rayquaza, Persian, Arceus & Ambipom all have a speed of 120 or less.
My opinion is that all things with roughly 120 could be a serious threat.
The serious threats/highest 10 speed are:
Deoxys S-Form (180 speed)
Ninjask (160 speed)
Deoxys Normal (150 speed)
Deoxys A-Form (150 speed)
Mega Alakazam (150 speed)
Mega Aerodactyl (150 speed)
Mega Beedrill (145 speed)
Mega Sceptile (145 speed)
Accelgor (145 speed)
Electrode (140 speed)
In reflection, average good would be 115-120 and above average would be 125+
Hope I answered your question ^<^