Spell Speed (Japanese: 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:
Spell Speed 1
Spell Speed 1 (Japanese: スペルスピード1) is the slowest of all Spell Speeds. These card effects cannot be Chained to the activation of a card or effect. Typically, these effects cannot be Chain Link 2 or higher, unless multiple Spell Speed 1 effects are activated simultaneously.
The following are all Spell Speed 1:
Normal Spell Card
Field Spell Card
Continuous Spell Card
Equip Spell Card
Ritual Spell Card
Monster Flip Effect
Monster Ignition Effect
Monster Trigger Effect
Spell Ignition-like Effect
Spell Trigger-like Effect
Spell Speed 2
Spell Speed 2 (Japanese: スペルスピード2) card effects are effects that can be used when you priority. Typically, you use these in response to Spell Speed 1 and other Spell Speed 2 effects, however some can start a Chain themselves as well. Spell Speed 2 card and effects cannot be activated in response to Spell Speed 3 effects. Spell Speed 2 effects are also called "Fast Effects".
If a card states, "during either player's turn", then it is considered Spell Speed 2. It may also say "(this is a Quick Effect)".
All Continuous Trap Cards are considered to have Spell Speed 2 effects, even if they may omit "during either player's turn".
Spell Speed 2 (Japanese: スペルスピード2) card effects are effects that can be used when you priority. Typically, you use these in response to Spell Speed 1 and other Spell Speed 2 effects, however some can start a Chain themselves as well. Spell Speed 2 card and effects cannot be activated in response to Spell Speed 3 effects. Spell Speed 2 effects are also called "Fast Effects".
If a card states, "during either player's turn", then it is considered Spell Speed 2. It may also say "(this is a Quick Effect)".
All Continuous Trap Cards are considered to have Spell Speed 2 effects, even if they may omit "during either player's turn". An example of this is the effect of Dark Smog.
The following are all Spell Speed 2:
Quick-Play Spell Card
Normal Trap Card
Continuous Trap Card
Monster Quick Effect
Trap Quick-like Effect
Spell Speed 3
Spell Speed 3 (Japanese: スペルスピード3) card effects are the "fastest" of all card effects; only other Spell Speed 3 card effects can be used to respond to a Spell Speed 3 card effect. As such, they are also classified as Fast effects.
The only cards that classify as Spell Speed 3 are Counter Trap Cards.
Cards like:
Solemn warning
Solemn strike
solemn scolding
All these cards are counter
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 a card like "Bottomless Trap Hole" or "Mirror Force" is used in response to such an action, that card will be considered Chain Link 1.