Spell Speed refers to how "fast" a Card Effect resolves after activation. Even though its name tells otherwise, Spell Speed is not restricted to the Effects of Spell Cards, but rather applies to any Card Effect that can be activated.
There are three Spell Speeds:
Spell Speed 1
(Spell Cards except for Quick Play Spell Cards and Monster Effects)
Spell Speed 2
(Quick Play Spell Cards, Trap Cards except for Counter Trap Cards and Quick Effect Monster Effects)
Spell Speed 3
(Counter Trap Cards)
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.
For example, if you control two “Black Whirlwinds” and Normal Summon a “Blackwing Monster” both “Black Whirlwinds” can be chained to make one “Black Whirlwind” Chain Link 2.
Actions without Spell Speed
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" is used in response to Summoning a Monster, it will be considered Chain Link 1.