Spell Speed (スペルスピード, Superusupīdo) refers to how "fast" a card effect resolves after activation. Despite their name, they are not restricted to the effects of Spell Cards, but rather apply to any card effect that can activate. There are 3 Spell Speeds:
When a card effect activates on a Chain, only effects with an equal or greater Spell Speed can respond to it. However, Spell Speed 1 effects cannot be used in response to another Spell Speed 1 effect. The only time two Spell Speed 1 effects can exist on the same Chain (and the only time Spell Speed 1 effects can be anything higher than Chain Link 1) is if both effects activate at exactly the same time due to some external factor; see Simultaneous Effects Go On Chain for more explanation.
Summoning a monster and declaring an attack are not card effects and do not "activate", therefore they do not have a Spell Speed and do not start a Chain. If an card like "Bottomless Trap Hole" or "Mirror Force" is used in response to such an action, that card will be considered Chain Link 1.