Dodge and hit

I am a bit confused about dodging and hitting via the speed stat. An example I found today was versus two monsters.

The first monster, a level 2100 sphinx avenger, with 687 speed.
The second monster, a level 1663 diabolic observer with around 84 thousand speed.

The diabolic observer attacked the sphinx with a basic attack command, and the sphinx dodged.

The sphinx had a sword artifact equipped with no dodge mechanic modifiers, and no dodge mechanic buffs were present when the attack took place.

My question is, how much speed is required to not be dodged when you make a basic attack with no modifiers?