Cooldown (or CD) is used in this game for all items that activate regularly, rather than "attack speed" or similar. CD represents "seconds per activation" (the inverse of "attacks per second" also used in many games). This means an item with a CD of 4 seconds will activate once every 4 seconds (or, 1/4=0.25, 0.25 times per second).
Following this logic further we get that a speed boost of +100% can be read as "activates twice as often". So a 4s CD becomes 2s with a +100% speed boost (or 0.25 activations per second becomes 0.5).
An easy way to differentiate different sources of speed boosts is to think about the modifiers in these categories:
- "internal" such as Flute ⭐ or Food ⭐ bonuses.
- "external" such as Gloves of Haste/Fanny Pack/Jynx torquilla/Mr. Struggles etc.
- "in battle" such as Heat or Cold.
Each category add with others in the same category, but multiply with those outside.
Formula:
Base CD / ("internal" x "external" x "in battle")
For each category all the speed boosts should be added together and then add 1. Thus a +20% speed boost is a CD factor of 1.2.
Example:
A Pineapple with 4s Base CD, next to 3 other non-pineapple food items, boosted by 2 Fanny Packs, 1 Gloves of Haste and 1 Mr. Struggles (from being in it's 
slot).
Using the formula it's CD becomes: 4s / (1.3 x 2.8) = 4s / 3.64 = 1.09. A CD of 1.09 (or 1/1.09=0.91 activations per second) and a total of +264% speed.